Substantial sip of alcohol - a sign of nerves? (8)
I believe the answer is:
dramatic
'substantial' is the definition.
(I've seen this before)
'sip of alcohol a sign of nerves?' is the wordplay.
'sip of alcohol' becomes 'dram'.
'sign of nerves?' becomes 'tic'.
'dram'+'a'+'tic'='DRAMATIC'
(Other definitions for dramatic that I've seen before include "Sensational, thrilling" , "Striking, impressive" , "Substantial" , "might be staggering" , "Relating to theatre and acting" .)