Trendy friend swallows it with one at first (9)
I believe the answer is:
initially
'at first' is the definition.
(synonyms)
'trendy friend swallows it with one' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'friend' becomes 'ally' (an ally is a friend or supporter).
'swallows' is an insertion indicator.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'in'+'ally'='inally'
'it'+'i'='iti'
'inally' enclosing 'iti' is 'INITIALLY'.
(Other definitions for initially that I've seen before include "To begin with" , "At the start" , "In the beginning" , "How laser, for example, was put together" , "At first" .)