Yorkshire town certainly backed retaining pound (5)
I believe the answer is:
selby
'yorkshire town' is the definition.
(I've seen this before)
'certainly backed retaining pound' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'backed' is a reversal indicator.
'retaining' means one lot of letters goes inside another (inserted letters are held or retained).
'pound' becomes 'lb' (synonyms).
'yes' in reverse letter order is 'sey'.
'sey' enclosing 'lb' is 'SELBY'.
(Other definitions for selby that I've seen before include "Town in North Yorkshire" , "N Yorks town" , "Yorks abbey" .)