Good exercise channel (4)
I believe the answer is:
pipe
'channel' is the definition.
(pipe is a kind of channel)
'good exercise' is the wordplay.
'good' becomes 'pi' (I've seen this in other clues).
'exercise' becomes 'pe' (Physical Education).
'pi'+'pe'='PIPE'
(Other definitions for pipe that I've seen before include "Main provider" , "two hogsheads of wine" , "Tube with bowl for smoking tobacco" , "Tube, maybe musical" , "Means of conveying" .)