What part of speech is underspend?

Underspend can be categorized as a verb.

Parts of speech

1. underspend is a verb, present, 1st person singular of underspend (infinitive).
2. underspend is a verb (infinitive).

Verb
underspend - spend at less than the normal rate
underspend - spend less than the whole of (a budget, for example)
Noun
underspend - An amount by which an allocated amount is not expended.