A son, not very tall and rather dim (7)

I believe the answer is:

slowish

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'a son not very tall and rather' is the definition.
I can't tell whether this defines the answer.

'dim' is the wordplay.
I cannot really understand how this works, but
'dim' could be 'slow' ('slow' can be a synonym of 'dim') and 'slow' is found within the answer.
The remaining letters 'ish' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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