Support Liberal party at last: not to begin with (8)
I believe the answer is:
secondly
'not to begin with' is the definition.
I can't tell whether this definition defines the answer.
'support liberal party at last' is the wordplay.
'support' becomes 'second' (eg to second an idea is to support it).
'at last' indicates one should take the final letters.
The final letters of 'liberal party' are 'ly'.
'second'+'ly'='SECONDLY'
Can you help me to learn more?
(Other definitions for secondly that I've seen before include "Not in first place" , "Next after firstly" .)