Huge performance by worker in charge (8)
I believe the answer is:
gigantic
'huge' is the definition.
(synonyms)
'performance by worker in charge' is the wordplay.
'performance' becomes 'gig' (a gig is a musical concert).
'by' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'gig'+'ant'+'ic'='GIGANTIC'
(Other definitions for gigantic that I've seen before include "'Enormous, mammoth (8)'" , "walloping" , "monster" , "Huge, enormous" , "Sizeable" .)