It's only me again (4)
I believe the answer is:
mere
'it's only' is the definition.
(I've seen this before)
'me again' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'me'+'re'='MERE'
(Other definitions for mere that I've seen before include "Lake - French mother" , "Pool -- pure and simple" , "sea" , "Pool, small lake" , "Pond - pure and simple" .)