Once again pick up explosive put in tower (6)
I believe the answer is:
rehear
'once again pick up' is the definition.
('hear' can be a synonym for 'pick up' so perhaps 'rehear' is OK for 'once again pick up')
'explosive put in tower' is the wordplay.
'explosive' becomes 'he' (abbreviation for high explosive).
'put in' indicates putting letters inside.
'tower' becomes 'rear' (I've seen this in another clue).
'he' placed inside 'rear' is 'REHEAR'.
(Other definitions for rehear that I've seen before include "be a sympathetic listener, not for the first time" , "Try again (in court)" , "Retry (a lawsuit)" , "Hold the trial again." .)