An officer runs into unstable ground (9)
I believe the answer is:
subaltern
'an officer' is the definition.
(subaltern is a kind of officer)
'runs into unstable ground' is the wordplay.
'runs' becomes 'r' (abbreviation in cricket).
'into' is an insertion indicator.
'ground' indicates an anagram ('grind' the letters into a new form).
'unstable' with letters rearranged gives 'subalten'.
'r' put into 'subalten' is 'SUBALTERN'.
(Other definitions for subaltern that I've seen before include "Army officer below the rank of captain" , "Junior officer" , "Lesser official" .)