Favourite union leader left with worker, peevish (8)
I believe the answer is:
petulant
'peevish' is the definition.
(I know that peevish can be written as petulant)
'favourite union leader left with worker' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'leader' indicates taking the first letters.
'with' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant).
The initial letter of 'left' is 'l'.
'pet'+'u'+'l'+'ant'='PETULANT'
(Other definitions for petulant that I've seen before include "Querulous and demanding" , "Easily irritated" , "Fractious, pouty" , "Pouting" , "Easily provoked" .)