Across

• 1.

  NEIL ARMSTRONG: NEIL ARMSTRONG (Mr. Alien rev. + strong)
• 11.

  ERIE: ERIE (hidden)
• 12.

  CARDINAL: CARDINAL (double def.)
• 14.

  TUNNEL: TUNNEL (anag.)
• 15.

  ELBOW: ELBOW
• 16.

  WHISTLER: WHISTLER (anag. + w)
• 18.

  SONG: SO(methi)NG
• 19.

  FIRST: FIRST (first letters)
• 21.

  FRANCIS BACON: FRANC-IS BACON
• 25.

  OBEY: O-BE-Y
• 28.

  MATURER: MAT-UR-ER (mater around our "but [for the] first")
• 30.

  ALBUM: A-LB-UM
• 33.

  SPIRE: SP-IRE
• 34.

  ATLAS: ATLAS (double def.)
• 35.

  EDGE: EDGE (double def.)
• 37.

  FRAGMENTS: FRAGMENTS (anag.)
• 40.

  COURTED: COURTED (true anag. in cod)
• 41.

  WADDLE: WADDLE (re-headed saddle)
• 42.

  FAME: FAME (hidden)
• 43.

  WHITEHEAD: WHITE-HEAD
• 44.

  FINALE: FIN-ALE
• 45.

  MITRE: MI(T)RE
• 46.

  SIR FRANCIS DRAKE: SIR FRANCIS DRAKE (anag.)

Down

• 1.

  CHECKERS: CHECKERS (double def.)
• 2.

  ORNERY: ORNERY (hidden)
• 3.

  NINES: NINES (double def.)
• 4.

  MEET: MEET (double def.)
• 5.

  LAZE: LAZE (homophone)
• 6.

  IDLE: IDLE (anag.)
• 7.

  NIBS: NIBS (double def.)
• 8.

  MATA HARI: MATA HARI (anag.)
• 9.

  FAWN: FAWN (double def.)
• 10.

  MARCIANO: MARCIANO (anag.)
• 12.

  CLIPPER: CLIPPER (double def.)
• 13.

  RETRO: RETRO (terro[rists] anag.)
• 17.

  FREUD: FREUD (fraud with "change of heart")
• 19.

  FEMME: FE(MM)E
• 20.

  THRIFT: TH-RIFT
• 22.

  YELL: YELL (with ow is "cowardly")
• 23.

  NIETZSCHE: NIETZSCHE (anag.)
• 24.

  TREADLED: TREAD-LED
• 26.

  BABE RUTH: BABE RUTH
• 27.

  HUSTLERS: HUSTLERS (anag.)
• 29.

  EARL OF SANDWICH: EARL OF SANDWICH (anag.)
• 31.

  BANDIT: BANDIT (homophonic pun)
• 32.

  MOSEYED: MOS-EYED
• 36.

  GUANO: GUANO (hidden rev.)
• 38.

  REBEL: REBEL (re instead of la on "label")
• 39.

  EDDIC: EDDIC (hidden)
• 40.

  CAFE: CA-FE