Seaman, mostly crazy, legendary strong man (6)
I believe the answer is:
tarzan
'legendary strong man' is the definition.
(character in Edgar Rice Burroughs' Tarzan books)
'seaman mostly crazy' is the wordplay.
'seaman' becomes 'tar' ('tar' can be a synonym of 'seaman').
'mostly' means to remove the last letter.
'crazy' becomes 'zany' (I've seen this before).
'zany' with its final letter taken off is 'zan'.
'tar'+'zan'='TARZAN'
(Other definitions for tarzan that I've seen before include "E. R. Burroughs' hero" , "Hero of Edgar Rice Burroughs novels" , "Edgar Rice Burroughs's fictional apeman" , "Edgar Rice Burroughs's man of the jungle" , "Fictional hero raised by apes" .)