Citrus fruit essential when climbing (7)
I believe the answer is:
satsuma
'citrus fruit' is the definition.
(satsuma is a kind of citrus fruit)
'essential when climbing' is the wordplay.
'essential' becomes 'a must' (it's a must).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'amust'+'as'='amustas'
'amustas' in reverse letter order is 'SATSUMA'.
(Other definitions for satsuma that I've seen before include "Segmented item" , "Small, easily-peeled orange" , "Japanese province and small orange" , "Type of mandarin orange" , "that's segmented" .)