Mate, I nearly forgot to back more than one English runner (6)
I believe the answer is:
spouse
'mate' is the definition.
(I know that mate can be written as spouse)
'i nearly forgot to back more than one english runner' is the wordplay.
'i nearly forgot' becomes 'PS' (as in a PS in a letter).
'to back' is a reversal indicator.
'more than one english runner' becomes 'Ouse' (name of several English rivers - rivers run).
'ps' in reverse letter order is 'sp'.
'sp'+'ouse'='SPOUSE'
(Other definitions for spouse that I've seen before include "Husband or wife" , "Other half" , "Partner in marriage" , "mate" , "Possibly wife" .)