Coastguard, perhaps, will tell you to look out for barrier (7)
I believe the answer is:
seawall
'barrier' is the definition.
(I know that seawall is a type of barrier)
'coastguard perhaps will tell you to look out' is the wordplay.
I cannot really understand how this works, but
'will' could be 'll' (contracted form, as in 'he'll' - 'he will') and 'll' is located in the answer.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Another definition for seawall that I've seen is " sea defence".)