Drain to remove contains pressure (7)
I believe the answer is:
deplete
'drain' is the definition.
(I know that drain is a kind of deplete)
'remove contains pressure' is the wordplay.
'remove' becomes 'delete' (deleting is a kind of removing).
'contains' indicates putting letters inside.
'pressure' becomes 'p' (scientific abbreviation).
'delete' enclosing 'p' is 'DEPLETE'.
'to' is the link.
(Other definitions for deplete that I've seen before include "Drain" , "Reduce the number of" , "Use up quantities of, reduce store" , "bankrupt" , "Spend" .)