Performers in Gigi principally had to dress down (9)
I believe the answer is:
castigate
'dress down' is the definition.
(castigating is a kind of dressing down)
'performers in gigi principally had' is the wordplay.
'performers' becomes 'cast' (I've seen this before).
'principally' suggests taking the first letters.
'had' becomes 'ate' ('have' can mean 'eat').
The initial letters of 'in gigi' is 'ig'.
'cast'+'ig'+'ate'='CASTIGATE'
'to' acts as a link.
(Other definitions for castigate that I've seen before include "Punish" , "Censure severely" , "Blast" , "Chastise - tacit sage (anag)" , "Severely reprimand" .)