Sadly depleted by 550, let's suppose (3)
I believe the answer is:
say
'suppose' is the definition.
(I know that suppose can be written as say)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for say that I've seen before include "Authority to influence decision" , "Opportunity to influence < > state" , "Verbalise" , "Utter; e.g." , "Aver" .)