Plumbers head in to remove drain (7)
I believe the answer is:
deplete
'drain' is the definition.
(I know that drain is a more specific form of the action deplete)
'plumbers head in to remove' is the wordplay.
'head' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'to remove' becomes 'delete' (deleting is a kind of removing).
The initial letter of 'plumbers' is 'p'.
'p' placed inside 'delete' is 'DEPLETE'.
(Other definitions for deplete that I've seen before include "bankrupt" , "Use up, reduce in quantity" , "Drain" , "Reduce (amount of supplies etc)" , "Spend" .)