Furious sailor gets in free (5)
I believe the answer is:
rabid
'furious' is the definition.
(thesaurus)
'sailor gets in free' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'gets in' indicates putting letters inside.
'free' becomes 'rid' ('rid' can be a synonym of 'free').
'ab' going into 'rid' is 'RABID'.
(Other definitions for rabid that I've seen before include "Crazy" , "Ferocious" , "Extremely fanatical" , "Raging (having been bitten by a dog?)" , "Burning" .)