Current number opening paper otherwise (2,3)
I believe the answer is:
if not
'otherwise' is the definition.
(synonyms)
'current number opening paper' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'number' becomes 'no' (abbreviation).
'opening' indicates putting letters inside (some letters open a gap in others).
'paper' becomes 'ft' (Financial Times newspaper).
'no' put inside 'ft' is 'fnot'.
'i'+'fnot'='IF NOT'
(Other definitions for if not that I've seen before include "Failing that" , "Unless" .)