Long for Glen to transform your old version (7)
I believe the answer is:
lengthy
'long' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lengthy that I've seen before include "Prolonged" , "Unusually long" , "Of considerable duration" , "Taking up a lot of time" , "Rather long" .)