One allowed to catch finale of Jonathan Creek (5)
I believe the answer is:
inlet
'creek' is the definition.
(I've seen this before)
'one allowed to catch finale of jonathan' is the wordplay.
'one' becomes 'i' (Roman numeral).
'allowed to' becomes 'let' (to let something happen is to allow it).
'catch' means one lot of letters goes inside another.
'finale of' says to take the final letters.
The final letter of 'jonathan' is 'n'.
'i'+'let'='ilet'
'ilet' enclosing 'n' is 'INLET'.
(Other definitions for inlet that I've seen before include "Sound" , "Gulf" , "Opening - passage" , "Bay or recess" , "Small coastal opening" .)