Debate finally politician secured, becoming exhausted (7)
I believe the answer is:
emptied
'exhausted' is the definition.
(I've seen this before)
'debate finally politician secured' is the wordplay.
'finally' suggests the final letters.
'politician' becomes 'MP' (Member of Parliament).
'secured' becomes 'tied' (tying is a kind of securing).
The final letter of 'debate' is 'e'.
'e'+'mp'+'tied'='EMPTIED'
'becoming' is the link.
(Other definitions for emptied that I've seen before include "Drained" , "Evacuated" , "having nothing more to offer" , "given vacation" , "Vacated" .)