Fussy people with power biting insect's head off! (7)
I believe the answer is:
pedants
'fussy people' is the definition.
(pedants are fussy)
'power biting insect's head off' is the wordplay.
'power' becomes 'p' (symbol in physics).
'biting' becomes 'eda' (I can't justify this - if you can you should give a lot more credence to this answer).
'insect's' becomes 'ant's' (ant is a kind of insect).
'head off' means to remove the first letter (the first letter or 'head' is removed).
'ants' with its initial letter removed is 'nts'.
'p'+'eda'+'nts'='PEDANTS'
'with' is the link.
(Other definitions for pedants that I've seen before include "They are over-concerned with correctness of detail" , "More than one lover of accuracy" , "Paraders of knowledge" .)