Fool! Exercise outside is well and truly out of date! (5)
I believe the answer is:
passe
'out of date' is the definition.
(I've seen this in another clue)
'fool exercise outside' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'exercise' becomes 'pe' (Physical Education).
'outside' indicates putting letters inside.
'ass' placed inside 'pe' is 'PASSE'.
'is well and truly' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for passe that I've seen before include "Outdated" , "dated" , "Jaded" , "Master key - picture mounting" , "Old hat" .)