Liberal wearing tousled hair promises to be funny (9)
I believe the answer is:
hilarious
'be funny' is the definition.
(thesaurus)
'liberal wearing tousled hair promises' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'wearing' means one lot of letters goes inside another.
'tousled' indicates anagramming the letters.
'promises' becomes 'ious' (IOU's, promises to pay a debt).
'hair' anagrammed gives 'hiar'.
'l' going within 'hiar' is 'hilar'.
'hilar'+'ious'='HILARIOUS'
'to' acts as a link.
(Other definitions for hilarious that I've seen before include "A scream" , "Louis' hair! (anag)" , "Extremely funny" , "Priceless!" , "Extremely amusing" .)