It’s muddy where the lad’s holding the horse (5)

I believe the answer is:

boggy

'it's muddy' is the definition.
('boggy' can be a synonym of 'muddy')

'the lad's holding the horse' is the wordplay.
'the lad' becomes 'boy' (I've seen this before).
'holding' means one lot of letters goes inside another.
'the horse' becomes 'gg'.
'boy' enclosing 'gg' is 'BOGGY'.

'where' is the link.

(Another definition for boggy that I've seen is " Moorish".)

I've seen this clue in the Evening Standard.

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