Small bolt one's installed for a collector (8)
I believe the answer is:
squirrel
'small' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for squirrel that I've seen before include "Tree-climbing rodent with bushy tail, red or grey" , "Tree-climbing rodent which hoards nuts" , "Arboreal rodent with long bushy tail" , "Brown or red rodent" , "Bushy-tailed tree dweller, red or grey" .)