Proposal to remove front of strongbox (5)
I believe the answer is:
offer
'proposal' is the definition.
(I've seen this in other clues)
'to remove front of strongbox' is the wordplay.
'to remove front of' means to remove the first letter (I've seen 'to remove front' mean this).
'strongbox' becomes 'coffer' (I've seen this before).
'coffer' with its initial letter taken away is 'OFFER'.
(Other definitions for offer that I've seen before include "Tender - proposal" , "Proposal (of marriage)" , "Present to or make a bid" , "Hold out, tender" , "Express willingness - present" .)