Protesting strongly against fence (7)
I believe the answer is:
railing
'protesting' is the definition.
(to rail against something is to protest it)
'strongly against fence' is the wordplay.
'strongly' becomes 'ing' (I can't justify this - if you can you should believe this answer much more).
'against' means one lot of letters go next to another.
'fence' becomes 'rail fence' (rail fence is a kind of fence).
'ing' after 'rail' is 'RAILING'.
Can you help me to learn more?
(Other definitions for railing that I've seen before include "Barrier, balustrade" , "Mocking" , "Fence of metal bars" , "Complaining bitterly - barrier" , "Fulminating against the barrier?" .)