Altering navy rig at sea (7)
I believe the answer is:
varying
'altering' is the definition.
('vary' can be a synonym of 'alter')
'navy rig at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'navy'+'rig'='navyrig'
'navyrig' is an anagram of 'VARYING'.
(Other definitions for varying that I've seen before include "Differing, diversifying" , "Differing, changing" .)