It causes a hold-up in some bottleneck (7)
I believe the answer is:
stopper
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'a hold-up in some bottleneck' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
'hold' could be 'stop' (stop is a kind of hold) and 'stop' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'causes' acts as a link.
Can you help me to learn more?
(Other definitions for stopper that I've seen before include "Perhaps cork" , "Plug - cork" , "One blocks" , "Cork - goalkeeper?" , "One who checks" .)