Fairly big rope securing rig at sea (7)
I believe the answer is:
largish
'fairly big' is the definition.
(I've seen this before)
'rope securing rig at sea' is the wordplay.
'rope' becomes 'lash'.
'securing' is an insertion indicator.
'at sea' indicates an anagram.
'rig' is an anagram of 'rgi'.
'lash' placed around 'rgi' is 'LARGISH'.
(Other definitions for largish that I've seen before include "Fairly big" , "Comfortable shoe" , "Somewhat hefty" , "Of considerable extent" , "sizeable proportions" .)