In fact, join a new league? (6)
I believe the answer is:
really
'in fact' is the definition.
(synonyms)
'join a new league?' is the wordplay.
I cannot quite see how this works, but
'join' could be 'ally' (I've seen this before) and 'ally' is located in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for really that I've seen before include "Actually, no kidding" , "Undoubtedly" , "Actually, honest to God" , "without a doubt" , "Dead" .)