A rare occasion, being depressed, to wander listlessly (4,4)
I believe the answer is:
blue moon
'a rare occasion' is the definition.
(once in a blue moon)
'depressed to wander listlessly' is the wordplay.
'depressed' becomes 'blue' (I've seen this before).
'to wander listlessly' becomes 'moon' (moon can mean to wander or mope around).
'blue'+'moon'='BLUE MOON'
'being' acts as a link.
(Other definitions for blue moon that I've seen before include "Very rarely, if once in one of these" , "Indicator of rarity" , "Lunar occurrence (which almost never happens)" , "In which once is very seldom" , "Once in one is a rare event" .)