Fight with proud leader, making them buckle (4)
I believe the answer is:
warp
'buckle' is the definition.
(I know that buckle can be written as warp)
'fight with proud leader' is the wordplay.
'fight' becomes 'war' (warring is a kind of fighting).
'with' means one lot of letters go next to another.
'leader' says to take the initial letters.
The initial letter of 'proud' is 'p'.
'war'+'p'='WARP'
'making them' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for warp that I've seen before include "Distort as wood may from heat" , "Crane fly larva" , "Make or become out of shape" , "Misintepret" , "Distort, ship's rope" .)