Mariner loves swimming free (7)
I believe the answer is:
absolve
'free' is the definition.
('absolve' can be a synonym of 'free')
'mariner loves swimming' is the wordplay.
'mariner' becomes 'ab'.
'swimming' is an anagram indicator.
'loves' anagrammed gives 'solve'.
'ab'+'solve'='ABSOLVE'
(Other definitions for absolve that I've seen before include "Grant remission of a sin" , "Let off the hook" , "Fully pardon" , "Set free from guilt" , "Lob vase (anag.)" .)