Best gig turned out the least small (7)
I believe the answer is:
biggest
'the least small' is the definition.
('big' is the opposite of 'small' so 'biggest' should be a synonym for 'least small')
'best gig turned' is the wordplay.
'turned' indicates anagramming the letters.
'best'+'gig'='bestgig'
'bestgig' with letters rearranged gives 'BIGGEST'.
'out' acts as a link.
(Other definitions for biggest that I've seen before include "Largest" , "Extremely great" , "Greatest in size" , "Superlatively large" .)