Trendy chap takes starting point seriously (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(synonyms)
'trendy chap takes starting point' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'chap' becomes 'ernest' ().
'takes' indicates putting letters inside.
'starting point' becomes 'A' (as in the phrase 'get from A to B').
'ernest' placed around 'a' is 'earnest'.
'in'+'earnest'='IN EARNEST'
(Other definitions for in earnest that I've seen before include "with sincere intentions" , "with serious intent" , "Purposefully" , "Not joking" , "with enthusiasm" .)