Spare material certainly running short and not entirely lavish (7)
I believe the answer is:
surplus
'spare material' is the definition.
(leftover or excess material)
'certainly running short and not entirely lavish' is the wordplay.
'certainly' becomes 'sure' (synonyms).
'running short' means to remove the last letter.
'and' says to put letters next to each other.
'not entirely' means to remove the last letter (not all of the word).
'lavish' becomes 'plush' (synonyms).
'sure' with its final letter removed is 'sur'.
'plush' with its final letter taken away is 'plus'.
'sur'+'plus'='SURPLUS'
(Other definitions for surplus that I've seen before include "superfluity" , "balance" , "Remainder" , "Redundant" , "Surfeit" .)