Evergreen youth takes exercise for nothing (7)
I believe the answer is:
'evergreen' is the definition.
(I've seen this before)
'youth takes exercise for nothing' is the wordplay.
I cannot quite understand how this works, but
'exercise' could be 'pe' (Physical Education) and 'pe' is found in the answer.
This explanation may well be incorrect...
(Other definitions for juniper that I've seen before include "Its cones are used to flavour gin" , "Plant producing berries used to flavour gin" , "Evergreen shrub or small tree with berry-like cones" , "Shrub flavouring gin" , "Shrub with berries used to flavour gin" .)