A half of lager is unimportant (9)
I believe the answer is:
smallbeer
'a half' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'lager is unimportant' is the wordplay.
I cannot quite understand how this works, but
'lager' could be 'beer' (I've seen this before) and 'beer' is present in the answer.
'unimportant' could be 'small' (similar in meaning) and 'small' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for smallbeer that I've seen before include "something of no importance" , "A half pint, say" .)