Agreement from number overwhelmed by strength (6)
I believe the answer is:
assent
'agreement' is the definition.
(assent is a kind of agreement)
'number overwhelmed by strength' is the wordplay.
'number' becomes 'n'.
'overwhelmed by' means one lot of letters goes inside another (I've seen this in other clues).
'strength' becomes 'asset' (I've seen this before).
'n' put into 'asset' is 'ASSENT'.
'from' acts as a link.
(Other definitions for assent that I've seen before include "Compliance, agreement" , "Express approval or agreement" , "Acquiescence" , "Give consent" , "Accede, acquiesce" .)