Mad to free the imprisoned sailor (5)
I believe the answer is:
rabid
'mad' is the definition.
(I've seen this before)
'free the imprisoned sailor' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'the imprisoned' is an insertion indicator.
'sailor' becomes 'ab' (abbreviation for able seaman).
'rid' going around 'ab' is 'RABID'.
'to' acts as a link.
(Other definitions for rabid that I've seen before include "Raging (having been bitten by a dog?)" , "Violent; hydrophobic" , "Ferocious" , "Zealous" , "Furious like mad dog?" .)