'Clash' group's approaching start of gig (5)
I believe the answer is:
clang
'clash' is the definition.
('clang' can be a synonym of 'clash')
'group's approaching start of gig' is the wordplay.
'group' becomes 'clan' (clan is a kind of group**).
'approaching' is a charade indicator (letters next to each other).
'start of' says to take the initial letters.
The first letter of 'gig' is 'g'.
'clan'+'g'='CLANG'
(Other definitions for clang that I've seen before include "Metallic ring" , "Loud resonant metallic sound" , "A loud ringing sound" , "Resounding effect" , "Noise from crane" .)