At first they were all nervy — finally showing pluck (5)
I believe the answer is:
twang
'pluck' is the definition.
(as in twanging or plucking a guitar string)
'at first they were all nervy finally showing' is the wordplay.
'at first' says to take the initial letters.
'finally' indicates one should take the final letters.
The first letters of 'they were all nervy' is 'twan'.
The last letter of 'showing' is 'g'.
'twan'+'g'='TWANG'
(Other definitions for twang that I've seen before include "Sort of sound" , "Regional intonation" , "A nasal intonation in speech, or sound of guitar" , "Local intonation" , "Harsh sound of plucked string" .)