Nonsense gets a weak laugh (6)
I believe the answer is:
guffaw
'laugh' is the definition.
(guffawing is a kind of laughing)
'nonsense gets a weak' is the wordplay.
'nonsense' becomes 'guff' (I've seen this before**).
'gets' says to put letters next to each other.
'weak' becomes 'w' (physics symbol as in W-boson).
'guff'+'a'+'w'='GUFFAW'
(Other definitions for guffaw that I've seen before include "Burst of loud, hearty laughter" , "Roar of laughter" , "Laugh heartily" , "A loud hearty laugh" , "Laugh boisterously" .)