Son to break down, being thin (6)
I believe the answer is:
sparse
'being thin' is the definition.
('sparse' can be a synonym of 'thin')
'son to break down' is the wordplay.
'son' becomes 's'.
'to break down' becomes 'parse' (parsing is a kind of breaking down).
's'+'parse'='SPARSE'
(Other definitions for sparse that I've seen before include "Thinly dispersed" , "Not much" , "Meagre, thin on the ground" , "Thinly-spread" , "Infrequent" .)