Trendy chap takes answer seriously (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(synonyms)
'trendy chap takes answer' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'chap' becomes 'ernest' ().
'takes' is an insertion indicator.
'answer' becomes 'a' (abbreviation eg in Q and A).
'ernest' placed around 'a' is 'earnest'.
'in'+'earnest'='IN EARNEST'
(Other definitions for in earnest that I've seen before include "Purposefully" , "Sincere in intention" , "Not joking" , "resolutely" , "Sincere and serious" .)