Thin poles collapse finally (6)
I believe the answer is:
sparse
'thin' is the definition.
(I know that thin can be written as sparse)
'poles collapse finally' is the wordplay.
'poles' becomes 'spars ' (spar is a kind of pole).
'collapse finally' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'spars'+'e'='SPARSE'
Can you help me to learn more?
(Other definitions for sparse that I've seen before include "Thinly dotted about" , "Not much" , "rarely occurring" , "Rare" , "Thin on the ground, scarce" .)