Declare nothing: yearn (5)
I believe the answer is:
opine
'declare' is the definition.
(I know this)
'nothing yearn' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'yearn' becomes 'pine' (synonyms).
'o'+'pine'='OPINE'
(Other definitions for opine that I've seen before include "Consider" , "State one's held view" , "Express one's opinion openly" , "Suggest" , "Express one's view" .)