Poles leave, finally thin on the ground (6)
I believe the answer is:
sparse
'thin on the ground' is the definition.
(I've seen this before)
'poles leave finally' is the wordplay.
'poles' becomes 'spars' (spar is a kind of pole).
'finally' indicates one should take the final letters.
The last letter of 'leave' is 'e'.
'spars'+'e'='SPARSE'
(Other definitions for sparse that I've seen before include "Thinly distributed" , "Meagre; scarce" , "Rare" , "One's not at all thick" , "not packed together" .)