Water supply needed by land near building -- large increase (11)
I believe the answer is:
groundswell
'large increase' is the definition.
(increase in popular opinion)
'water supply needed by land near building' is the wordplay.
'water supply' becomes 'well' (a well is a source of water).
'needed by' means one lot of letters go next to another.
'land near building' becomes 'grounds' (area around a building).
'well' after 'grounds' is 'GROUNDSWELL'.
(Other definitions for groundswell that I've seen before include "Forming opinion" , "that might make waves" , "gathering movement" , "Build-up of public opinion" , "pressure of public opinion" .)