Volunteers turn out medium fit (7)
I believe the answer is:
tantrum
'fit' is the definition.
(I know that fit can be written as tantrum)
'volunteers turn out medium' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'medium' becomes 'm' (as in UK clothing sizes).
'turn' with letters rearranged gives 'ntru'.
'ta'+'ntru'+'m'='TANTRUM'
(Other definitions for tantrum that I've seen before include "fit of bad temper" , "Childish outbreak of temper" , "Frenzy" , "throwing this?" , "Burst of rage" .)