... small containers containing ale finally? Not exactly ... (6)
I believe the answer is:
'not exactly' is the definition.
I can't judge whether this definition defines the answer.
'small containers containing ale finally?' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'containers' becomes 'tins' (tin is a kind of container**).
'containing' is an insertion indicator.
'finally?' says to take the final letters (I've seen 'finally' mean this).
The last letter of 'ale' is 'e'.
'tins' enclosing 'e' is 'teins'.
Can you help me to learn more?
(Another definition for steins that I've seen is " Large beer mugs".)