One in armchair perhaps, a learner finding fault (8)
I believe the answer is:
critical
'finding fault' is the definition.
(I've seen this before)
'one in armchair perhaps a learner' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'in' indicates putting letters inside.
'armchair perhaps' becomes 'critic' (I am not sure about this - if you are sure you should believe this answer much more).
'a learner' becomes 'l' (as in a learner driver with L-plates on their car).
'critic'+'l'='criticl'
'a' placed inside 'criticl' is 'CRITICAL'.
Can you help me to learn more?
(Other definitions for critical that I've seen before include "Decisive - grave" , "like one in an armchair perhaps" , "Captious, carping or crucial" , "Looking for faults, or at important point" , "Adversely commenting" .)