Mark mobile soldiers' second uprising (4)
I believe the answer is:
scar
'mark' is the definition.
(mark left by an injury)
'mobile soldiers' second uprising' is the wordplay.
'mobile soldiers'' becomes 'RAC' (Royal Armoured Corps, soldiers using tanks).
'second' becomes 's' (common abbreviation).
'uprising' is a reversal indicator.
'rac'+'s'='racs'
'racs' back-to-front is 'SCAR'.
(Other definitions for scar that I've seen before include "Bare craggy rock" , "Old wound - bare cliff" , "Mark left by old wound" , "Stigma" , "reminder of sword-fight?" .)