Sum needed to get garage built (9)
I believe the answer is:
aggregate
'sum' is the definition.
(aggregate is a kind of sum)
'get garage built' is the wordplay.
'built' indicates anagramming the letters (the same letters can be used to build the answer).
'get'+'garage'='getgarage'
'getgarage' is an anagram of 'AGGREGATE'.
'needed to' is the link.
(Other definitions for aggregate that I've seen before include "Combine into a whole" , "Get garage to do sum" , "Sum of many heterogenous things taken together" , "Accumulate into total" , "piece of granite perhaps" .)