Exercise outside to compress muscles (4)

I believe the answer is:

pecs

'muscles' is the definition.
(short for pectoral muscles)

'exercise outside to compress' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'outside to' says to hollow out the word (remove centre letters).
'compress' with its middle removed is 'cs'.
'pe'+'cs'='PECS'

I've seen this clue in The Telegraph.

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