Suppose it's nothing long (5)
I believe the answer is:
opine
'suppose' is the definition.
(I know that suppose can be written as opine)
'nothing long' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'long' becomes 'pine' (to pine for someone is to long for them).
'o'+'pine'='OPINE'
'it's' acts as a link.
(Other definitions for opine that I've seen before include "Believe" , "judge" , "Suggest" , "Suppose" , "State one's held view" .)