'Underspend' definitions:

Definition of 'underspend'

From: WordNet
verb
Spend less than the whole of (a budget, for example) [ant: overspend]
verb
Spend at less than the normal rate [ant: overspend]

Definition of 'Underspend'

From: GCIDE
  • Underspend \Un`der*spend"\, v. t. To spend less than. [1913 Webster]