Such muscles may be small in singers (7)

I believe the answer is:

tensors

'such muscles' is the definition.
(tensor is a type of muscle)

'small in singers' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'in' is an insertion indicator.
'singers' becomes 'tenors' (tenor is a kind of singer).
's' inserted inside 'tenors' is 'TENSORS'.

'may be' acts as a link.

I've seen this clue in The Telegraph.

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