Away representing Millwall, initially not playing well (3,4)
I believe the answer is:
off form
'not playing well' is the definition.
'away representing millwall initially' is the wordplay.
'away' becomes 'off' (synonyms).
'representing' becomes 'for' (as in a lawyer 'for' someone).
'initially' suggests taking the first letters.
The first letter of 'millwall' is 'm'.
'off'+'for'+'m'='OFF FORM'
(Other definitions for off form that I've seen before include "Describing indifferent quality" , "Not at one's best" , "playing badly" .)