Unemotional criticism, mostly involving nothing (5)
I believe the answer is:
stoic
'unemotional' is the definition.
(similar in meaning)
'criticism mostly involving nothing' is the wordplay.
'criticism' becomes 'stick' (giving someone stick is criticising them).
'mostly' means to remove the last letter (most but not all of the word).
'involving' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'stick' with its last letter taken off is 'stic'.
'stic' placed around 'o' is 'STOIC'.
(Other definitions for stoic that I've seen before include "Able to endure pain" , "One able to endure hardship; philosopher" , "One patient under misfortune" , "Ancient philosopher" , "One indifferent to fortune, pleasure and pain" .)