Fashion might start with poem (4)
I believe the answer is:
mode
'fashion' is the definition.
(synonyms)
'might start with poem' is the wordplay.
'start with' indicates taking the first letters.
'poem' becomes 'ode' (ode is a type of poem).
The first letter of 'might' is 'm'.
'm'+'ode'='MODE'
(Other definitions for mode that I've seen before include "Way of doing something" , "Way of operating" , "'Fashion, style (4)'" , "Manner of dome" , "Trend" .)