Fooled failure to take exercise (5)
I believe the answer is:
duped
'fooled' is the definition.
('dupe' can be a synonym of 'fool')
'failure to take exercise' is the wordplay.
'failure' becomes 'dud' (dud is a kind of failure).
'to take' means one lot of letters goes inside another (inserted letters are taken in).
'exercise' becomes 'pe' (Physical Education).
'dud' enclosing 'pe' is 'DUPED'.
(Other definitions for duped that I've seen before include "Fooled, codded" , "Deceived" , "Bamboozled" , "Taken for a ride" , "Tricked" .)