Meagre sum originally held in reserve (6)
I believe the answer is:
sparse
'meagre' is the definition.
(thesaurus)
'sum originally held in reserve' is the wordplay.
'originally' indicates taking the first letters.
'held in' is an insertion indicator.
'reserve' becomes 'spare' ('spare' can be a synonym of 'reserve'**).
The initial letter of 'sum' is 's'.
's' put within 'spare' is 'SPARSE'.
(Other definitions for sparse that I've seen before include "Thinly spread" , "rarely occurring" , "Thinly distributed" , "Thin on the ground, scarce" , "Meagre, scanty" .)