Tommy watches detectives (7,4)

I believe the answer is:

private eyes

'detectives' is the definition.
(a private eye is a private investigator)

'tommy watches' is the wordplay.
'tommy' becomes 'private' (informal term for a British soldier).
'watches' becomes 'eyes' (to eye is to watch or look at).
'private'+'eyes'='PRIVATE EYES'

I've seen this clue in The Telegraph.

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