Quick to show what you feel (7)
I believe the answer is:
express
'quick' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for express that I've seen before include "Fast train; specific" , "set forth" , "Utter - direct" , "air" , "Speak - fast" .)