Person looking down barrel had to play along (9)
I believe the answer is:
cooperate
'to play along' is the definition.
(to cooperate is to work along with others)
'person looking down barrel' is the wordplay.
I cannot quite see how this works, but
'person' could be 'rat' (rat is a kind of person) and 'rat' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'had' is the link.
Can you help me to learn more?
(Other definitions for cooperate that I've seen before include "Work constructively with others" , "work hand in glove" , "Work to the same end" , "Work with another, be helpful" , "Help" .)