Fanatical sailor free outside (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'sailor free outside' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'free' becomes 'rid' (to be free of something is to be rid of it).
'outside' is an insertion indicator.
'ab' inserted into 'rid' is 'RABID'.
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Zealous" , "Mad; raging" , "Raging (having been bitten by a dog?)" , "Extreme, fanatical" .)