Clerics' PR is one to let off steam (8)
I believe the answer is:
primates
'clerics' pr is one to let off steam' is the definition.
I can't judge whether this definition defines the answer.
'pr is one to let off steam' is the wordplay.
I cannot really see how this works, but
'pr' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
an anagram of 'steam' is 'mates' which is within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for primates that I've seen before include "Apt miser including archbishop and apes" , "Archbishops or apes" , "some leading churchmen" , "Animals including monkeys and apes" , "Apes; bishops" .)