One is a fool to take exercise (4)

I believe the answer is:

dope

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'one is a fool' is the definition.
I can't tell whether this definition defines the answer.

'take exercise' is the wordplay.
'take' becomes 'do' (eg taking or doing drugs).
'exercise' becomes 'pe' (Physical Education).
'do'+'pe'='DOPE'

'to' is the link.

Can you help me to learn more?

(Other definitions for dope that I've seen before include "cheat thereat?" , "Dolt -- drug" , "Information; nitwit" , "Information, fool" , "Silly ass takes drugs" .)

I've seen this clue in The Times.

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