Run hot iron on top for reason (5)
I believe the answer is:
infer
'for reason' is the definition.
(both can mean to deduce)
'run hot iron on top' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'hot' becomes 'in' (both can mean fashionable).
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'on top' says to put letters next to each other (in a down clue, some letters go above others).
'in'+'fe'='infe'
'r' after 'infe' is 'INFER'.
(Other definitions for infer that I've seen before include "Establish by deduction" , "Imply" , "Draw meaning from" , "Draw a conclusion" , "Take finer meaning from" .)