Marry groom and fashionable bride, finally (7)
I believe the answer is:
combine
'marry' is the definition.
(I've seen this before)
'groom and fashionable bride finally' is the wordplay.
'groom' becomes 'comb' (combing is a kind of grooming).
'and' says to put letters next to each other.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'finally' says to take the final letters.
The last letter of 'bride' is 'e'.
'comb'+'in'+'e'='COMBINE'
(Other definitions for combine that I've seen before include "Mix together different elements" , "Unite, merge" , "Join together" , "Blend" , "marry" .)