Go up to Heather leering (6)
I believe the answer is:
ogling
'leering' is the definition.
(I've seen this before)
'go up to heather' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'to heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'go' in reverse letter order is 'og'.
'og'+'ling'='OGLING'
(Other definitions for ogling that I've seen before include "Eyeing greedily" , "Leering" , "Looking lecherously" , "Eyeing lasciviously" .)