Gun full of rust, it did initially jam (8)
I believe the answer is:
gridlock
'jam' is the definition.
(I've seen this before)
'gun full of rust it did initially' is the wordplay.
'gun' becomes 'glock' (**).
'full of' indicates putting letters inside.
'initially' says to take the initial letters.
The initial letters of 'rust it did' is 'rid'.
'glock' enclosing 'rid' is 'GRIDLOCK'.
(Other definitions for gridlock that I've seen before include "Total traffic jam" , "Standstill" , "Traffic jam where no movement is possible" , "Severe traffic congestion" , "Dense, motionless traffic" .)