Prolonged start of hibernation gently refreshed bears (7)
I believe the answer is:
lengthy
'prolonged' is the definition.
(I know that prolonged can be written as lengthy)
'start of hibernation gently refreshed bears' is the wordplay.
'start of' indicates taking the first letters.
'refreshed' is an anagram indicator.
'bears' is an insertion indicator (some letters carry or bear others).
The initial letter of 'hibernation' is 'h'.
'gently' is an anagram of 'lengty'.
'h' placed into 'lengty' is 'LENGTHY'.
(Other definitions for lengthy that I've seen before include "Unusually long" , "Prolix - tedious" , "Rather long" , "rambling" , "Protracted" .)