Worker good when in better shape? (3-6)

I believe the answer is:

gas fitter

'worker' is the definition.
(gas fitter is a kind of worker)

'good when in better shape?' is the wordplay.
'good' becomes 'g' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'in better shape?' becomes 'fitter' (I've seen this before).
'g'+'as'+'fitter'='GAS-FITTER'

I've seen this clue in The Telegraph.

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