The main enemies of Doctor Glen King succeeded (6)
I believe the answer is:
daleks
'the main enemies of doctor' is the definition.
I know nothing about this answer so I can't judge whether this works.
'glen king succeeded' is the wordplay.
'glen' becomes 'dale' (I've seen this in another clue).
'king' becomes 'K' (abbreviation for king in chess).
'succeeded' becomes 's' (genealogical abbreviation).
'dale'+'k'+'s'='DALEKS'
Can you help me to learn more?
(Other definitions for daleks that I've seen before include "Doctor Who baddies" , "Dr Who's main enemies" , "Who fights these" , "Enemies in 13 ac" , "Major adversary" .)