Go for a long walk - or take up afternoon painting (5)
I believe the answer is:
tramp
'go' is the definition.
(I know that tramp is a kind of the walk)
'up afternoon painting' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'afternoon' becomes 'pm'.
'painting' becomes 'art' (painting is an art).
'pm'+'art'='pmart'
'pmart' reversed gives 'TRAMP'.
'for a long walk or take' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tramp that I've seen before include "Vagabond, hobo" , "Down-and-out" , "I might hitch a lift" , "Merchant ship" , "sometimes harboured?" .)