Marine creature circling large water barrier? (7)
I believe the answer is:
sealant
'water barrier?' is the definition.
(sealant might be used as a barrier against water)
'marine creature circling large' is the wordplay.
'marine' becomes 'sea ' (both can mean 'of the sea').
'creature' becomes 'ant' (ant is a kind of creature).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'large' becomes 'l'.
'sea'+'ant'='seaant'
'seaant' enclosing 'l' is 'SEALANT'.
(Other definitions for sealant that I've seen before include "closer?" , "Material used for an airtight or watertight join" , "Mastic maybe" , "Impervious gap filler" , "Material to prevent leakage" .)