Sort of idiom, initially not a word (3)
I believe the answer is:
ish
'sort of' is the definition.
(I've seen this in other clues)
'idiom initially not a word' is the wordplay.
'initially' suggests taking the first letters.
'not a word' becomes 'sh'.
The initial letter of 'idiom' is 'i'.
'i'+'sh'='ISH'
(Other definitions for ish that I've seen before include "Expiry (in Scots law)" , "loosely" , "Suffix meaning like or characteristic of" .)