Marine worker enclosing 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 worker enclosing large' is the wordplay.
'marine' becomes 'sea' (both can mean 'of the sea').
'worker' becomes 'ant' (worker is a type of ant).
'enclosing' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'sea'+'ant'='seaant'
'seaant' placed around 'l' is 'SEALANT'.
(Other definitions for sealant that I've seen before include "Substance filling gap to prevent leakage" , "Impervious gap filler" , "Material used for an airtight or watertight join" , "Filler used to close a gap" , "Eg, Fermat or Napier" .)