Be slow, so fail to finish a lager (3)
I believe the answer is:
lag
'be slow' is the definition.
(to lag is to be slow)
'fail to finish a lager' is the wordplay.
'to finish' suggests the final letters.
'a lager' becomes 'ag' (I can't justify this - if you can you should give a lot more credence to this answer).
The final letter of 'fail' is 'l'.
'l'+'ag'='LAG'
'so' acts as a link.
Can you help me to learn more?
(Other definitions for lag that I've seen before include "An old one is a convict" , "Fall behind; insulate" , "Delay" , "Dawdle" , "trail" .)