Swarms over nests if disturbed (7)
I believe the answer is:
infests
'swarms over' is the definition.
(I've seen this in another clue)
'nests if disturbed' is the wordplay.
'disturbed' is an anagram indicator (the order of letters is disturbed).
'nests'+'if'='nestsif'
'nestsif' is an anagram of 'INFESTS'.
(Other definitions for infests that I've seen before include "Fitness (anag.)" , "Set fins (anag.)" .)