Pertinent to affirm as a fact the monkey is outside (8)
I believe the answer is:
apposite
'pertinent' is the definition.
(I know that pertinent can be written as apposite)
'fact the monkey is outside' is the wordplay.
'fact' becomes 'posit' (I can't explain this - if you can you should believe this answer much more).
'the monkey' becomes 'ape' (I've seen this in other clues).
'is outside' means one lot of letters goes inside another (some letters go outside others).
'posit' inserted inside 'ape' is 'APPOSITE'.
'to affirm as a' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for apposite that I've seen before include "Appropriate, pertinent" , "Appropriate for purpose or occasion" , "Suitable, relevant" , "Pertinent, well-chosen" , "Suited for the purpose" .)