Admit formerly being taken in by three characters in succession? (7)
I believe the answer is:
concede
'admit' is the definition.
(I know that concede has the is a more specific form of the action admit)
'formerly being taken in by three characters in succession?' is the wordplay.
'formerly' becomes 'once' (synonyms).
'being taken in by' means one lot of letters goes inside another.
'three characters in succession?' becomes 'CDE' (consecutive letters in the alphabet).
'once' going within 'cde' is 'CONCEDE'.
(Other definitions for concede that I've seen before include "Yield; grant" , "Admit; yield" , "Allow - give up" , "Admit something is true" , "Acknowledge defeat" .)