At first they were all nervous: finally showing pluck (5)
I believe the answer is:
twang
'pluck' is the definition.
(twanging is a kind of plucking)
'at first they were all nervous finally showing' is the wordplay.
'at first' says to take the initial letters.
'finally' suggests the final letters.
The first letters of 'they were all nervous' is 'twan'.
The last letter of 'showing' is 'g'.
'twan'+'g'='TWANG'
(Other definitions for twang that I've seen before include "Local intonation" , "Sharp vibrating sound" , "Strongly nasal quality in speech" , "Nasal or musical sound" , "nasal effect" .)