Pest, unfortunately, elected to intervene (4,2)
I believe the answer is:
step in
'intervene' is the definition.
('step in' can be a synonym of 'intervene')
'pest unfortunately elected' is the wordplay.
'unfortunately' is an anagram indicator.
'elected' becomes 'in'.
'pest' is an anagram of 'step'.
'step'+'in'='STEP-IN'
'to' acts as a link.
(Other definitions for step in that I've seen before include "Type of garment" , "enter" , "Interrupt" , "Shoe with no fastenings" , "Substitute" .)