Alert cop massaged his back muscles (9)

I believe the answer is:

pectorals

'muscles' is the definition.
(pectoral is a kind of muscle)

'alert cop massaged his back' is the wordplay.
'massaged' is an anagram indicator (I've seen 'massaging' mean this ('massage' can mean to manipulate or distort)).
'back' says to take the final letters.
The final letter of 'his' is 's'.
'alert'+'cop'='alertcop'
'alertcop' anagrammed gives 'pectoral'.
'pectoral'+'s'='PECTORALS'

(Another definition for pectorals that I've seen is " Muscles".)

I've seen this clue in The Daily Express.

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