One pocketed by foolish client, I'd want double! (9,4)
I believe the answer is:
identical twin
'double' is the definition.
(I've seen this in another clue)
'one pocketed by foolish client i'd want' is the wordplay.
'one' becomes 'i' (Roman numeral).
'pocketed by' indicates putting letters inside.
'foolish' indicates anagramming the letters (a foolish spelling).
'client'+'id'+'want'='clientidwant'
'clientidwant' is an anagram of 'identcaltwin'.
'i' going within 'identcaltwin' is 'IDENTICAL TWIN'.
(Other definitions for identical twin that I've seen before include "Lookalike sibling" , "One of two from the same fertilised egg" .)