Legendary magician in summer line-up (6)
I believe the answer is:
merlin
'legendary magician' is the definition.
(I've seen this before)
'in summer line-up' is the wordplay.
'in' indicates a hidden word.
'MERLIN' is hidden in the letters of 'summer line up'.
(Other definitions for merlin that I've seen before include "Bird of prey - magician" , "Flighty type" , "Hawk-like magician of Camelot" , "Kind of hawk or Camelot's magician" , "King Arthur's magician" .)