Soldier beginning to goad weakest in group (5)
I believe the answer is:
grunt
'soldier' is the definition.
(grunt can mean a low-ranking soldier)
'beginning to goad weakest in group' is the wordplay.
'beginning to' suggests taking the first letters.
'weakest in group' becomes 'runt' (as in the runt of the litter).
The first letter of 'goad' is 'g'.
'g'+'runt'='GRUNT'
(Other definitions for grunt that I've seen before include "Sound made by a pig" , "Talk like a pig" , "Inarticulate sound" , "Noise of a pig ..." , "Beastly sound" .)