Associate stumbled and expressed pain (6)
I believe the answer is:
fellow
'associate' is the definition.
(I know that fellow is a type of fellow member)
'stumbled and expressed pain' is the wordplay.
'stumbled' becomes 'fell' (to stumble is to fall over).
'and' means one lot of letters go next to another.
'expressed pain' becomes 'ow' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fell'+'ow'='FELLOW'
Can you help me to learn more?
(Other definitions for fellow that I've seen before include "Comrade" , "Academic chap?" , "Colleague" , "Bloke; senior member of college" , "Boy or man" .)