Age about right, little jerk and friend in unreliable manner (11)
I believe the answer is:
erratically
'age about right little jerk and friend' is the definition.
I can't tell whether this defines the answer.
'unreliable manner' is the wordplay.
'unreliable' becomes 'erratic' (similar in meaning).
'manner' becomes 'ally' (I can't justify this - if you can you should give a lot more credence to this answer).
'erratic'+'ally'='ERRATICALLY'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for erratically that I've seen before include "Unpredictably" , "In an irregular manner" , "In a haphazard manner" .)