Most junior guy on set, surprisingly (8)
I believe the answer is:
youngest
'most junior' is the definition.
('young' is associated in meaning with 'junior')
'guy on set surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'guy'+'on'+'set'='guyonset'
'guyonset' with letters rearranged gives 'YOUNGEST'.
(Other definitions for youngest that I've seen before include "last born" , "baby" , "delivered most recently" , "Least elderly" , "least experienced" .)