Be fussy in dealing with infestation (7)
I believe the answer is:
nitpick
'be' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
(Other definitions for nitpick that I've seen before include "Find fault in petty way" , "Be fussily pedantic" , "Be very particular" , "Have little complaint" , "Criticise pettily" .)