Crossword Genius

Nervous person lowering head of beer in beer mug (9)

Ross

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" .)

I've seen this clue in The Times.
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