He's promised exercise will include shelf (7)

I believe the answer is:

pledgee

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'he's promised' is the definition.
The definition suggests a singular noun which matches the answer.

'exercise will include shelf' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'will include' means one lot of letters goes inside another.
'shelf' becomes 'ledge' (synonyms).
'pe' placed around 'ledge' is 'PLEDGEE'.

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(Other definitions for pledgee that I've seen before include "Uncle" , "One receiving promise" .)

I've seen this clue in The Daily Express.

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