Declare nothing with deal (5)
I believe the answer is:
opine
'declare' is the definition.
(I know that declare is a is a more specific form of the action of opine)
'nothing with deal' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'with' means one lot of letters go next to another.
'deal' becomes 'pine' (deal can mean wood for building with eg pine).
'o'+'pine'='OPINE'
(Other definitions for opine that I've seen before include "Conjecture" , "Believe" , "Say what one thinks" , "Hold forth" , "Consider" .)