Significant relief when donkey, say, gets a home (8)
I believe the answer is:
mountain
'significant relief' is the definition.
(relief can mean an area of higher ground)
'donkey say gets a home' is the wordplay.
'donkey say' becomes 'mount' (a rider might mount a donkey).
'gets' is a charade indicator (letters next to each other).
'home' becomes 'in' ('he's home' can mean 'he's in').
'mount'+'a'+'in'='MOUNTAIN'
'when' acts as a link.
(Other definitions for mountain that I've seen before include "Two-wheeler designed for rough terrain" , "Eg, K2" , "with pile of 1d [ADVENTURE]" , "may be in range?" , "Natural elevation of the earth's surface" .)