Nervous person lowering head of beer in beer mug (9)
I believe the answer is:
jitterbug
'nervous person' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
(Other definitions for jitterbug that I've seen before include "Nervous person" , "Fast, jerky American dance of the 1940s" , "1940s US dance" , "Lively jazz dance of the 1940s" .)