Limit on skill shown by ex-PM as a gardener (10,5)
I believe the answer is:
Here is my best explanation:
'a gardener' is the definition.
(famous English 18c gardener)
'limit on skill shown by ex-pm' is the wordplay.
'limit' becomes 'cap' (capping is a kind of limiting).
'on' is a charade indicator (letters next to each other).
'skill' becomes 'ability' (I've seen this before).
'shown by' says to put letters next to each other.
'ex-pm' becomes 'brown' (former UK PM Gordon Brown).
'as' is the link.