Only I count (6)
I believe the answer is:
merely
'only' is the definition.
(merely can mean only or simply)
'i count' is the wordplay.
'i' becomes 'me'.
'count' becomes 'rely' (I've seen this before).
'me'+'rely'='MERELY'
(Other definitions for merely that I've seen before include "'Only, and nothing more (6)'" , "Simply - only" , "Just one" .)