Create firm to medium stance (7)
I believe the answer is:
compose
'create' is the definition.
(composing is a kind of creating)
'firm to medium stance' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'to medium' becomes 'm' (as in UK clothing sizes).
'stance' becomes 'pose' (I've seen this in another clue).
'co'+'m'+'pose'='COMPOSE'
(Other definitions for compose that I've seen before include "Write a piece of music" , "restore calm" , "Devise" , "Create or put together" , "18 [GET READY]" .)