Frenchman’s duck, caught by a favourite opener (8)
I believe the answer is:
aperient
'opener' is the definition.
(drug which helps open one's bowels)
'frenchman's duck caught by a favourite' is the wordplay.
'frenchman's duck' becomes 'rien' ('nothing' in French - 'duck' means zero in cricket).
'caught by' indicates putting letters inside.
'favourite' becomes 'pet' (synonyms).
'a'+'pet'='apet'
'rien' inserted inside 'apet' is 'APERIENT'.
(Other definitions for aperient that I've seen before include "Mildly laxative" , "Laxative medicine - repeat in prescription" , "drug?" .)