Not slow as Liberal Left to put in a sympathetic word (3,5)
I believe the answer is:
all there
'not slow' is the definition.
(as opposed to 'not all there')
'liberal left to put in a sympathetic word' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'left' becomes 'l' (common abbreviation).
'to put in' indicates putting letters inside.
'sympathetic word' becomes 'there' (as in 'there, there').
'l'+'l'='ll'
'a'+'there'='athere'
'll' put within 'athere' is 'ALL THERE'.
'as' acts as a link.
(Other definitions for all there that I've seen before include "Not stupid (informal)" , "Shrewd" , "Everybody attending the event" , "Completely sane" , "rational" .)