Make slow progress when credit is no longer available (4,4)
I believe the answer is:
tick over
'make slow progress' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'credit is no longer available' is the wordplay.
'credit' becomes 'tick' (on tick can mean purchased on credit).
'is no longer available' becomes 'over' (I can't explain this - if you can you should give a lot more credence to this answer).
'tick'+'over'='TICK OVER'
'when' is the link.
Can you help me to learn more?
(Other definitions for tick over that I've seen before include "Run smoothly (or very slowly?)" , "Just keep going like idling engine" , "(Of engine) run slowly in neutral" , "(Of engine) idle" .)