Significantly increase nonsense about Liberal (6)
I believe the answer is:
triple
'significantly increase' is the definition.
(multiply by three)
'nonsense about liberal' is the wordplay.
'nonsense' becomes 'tripe' (informal term for nonsense or false information).
'about' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'tripe' placed around 'l' is 'TRIPLE'.
(Other definitions for triple that I've seen before include "involving several parties" , "Threefold, treble" , "make score sixty?" , "Multiply by three" , "In three parts" .)