What's found in tower? Try again! (6)
I believe the answer is:
rehear
'try again' is the definition.
('hear' can be a synonym for 'try' so perhaps 'rehear' is OK for 'try again')
'what's found in tower?' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'found in' indicates putting letters inside.
'tower?' becomes 'rear' (towering is a kind of rearing).
'eh' going within 'rear' is 'REHEAR'.
(Other definitions for rehear that I've seen before include "be a sympathetic listener, not for the first time" , "Hold the trial again." , "Try again (in court)" , "Retry (a lawsuit)" .)