Guy is light, having nothing on! (7)
I believe the answer is:
lampoon
'guy' is the definition.
(lampooning is a kind of guying)
'light having nothing on' is the wordplay.
'light' becomes 'lamp' (lamp is a kind of light).
'having nothing' becomes 'o' (I've seen this in other clues).
'lamp'+'o'+'on'='LAMPOON'
'is' acts as a link.
(Other definitions for lampoon that I've seen before include "Ridicule with satire" , "joke" , "send-up" , "Ridicule in writing or verse" , "Satirical personal attack" .)