Compliance in underwater action? (10)
I believe the answer is:
submission
'compliance' is the definition.
(I know that compliance can be written as submission)
'underwater action?' is the wordplay.
'underwater' becomes 'sub' (as in submarine).
'action?' becomes 'mission' (mission is a kind of action).
'sub'+'mission'='SUBMISSION'
'in' is the link.
(Other definitions for submission that I've seen before include "Surrender - written contribution to enquiry etc" , "Compliance" , "Giving up" , "presentation" , "Proposal" .)