Missing a symbol (5)
I believe the answer is:
minus
'missing' is the definition.
(thesaurus)
'a symbol' is the wordplay.
I cannot quite see how this works, but
'symbol' could be 'nu' (nu is a kind of symbol) and 'nu' is found in the answer.
The remaining letters 'mis' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for minus that I've seen before include "deficiency" , "Lacking; negative" , "Negative arithmetic operation" , "(Of temperature) below zero" , "Lacking; below zero" .)