Dark lake flanked by group of soldiers (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'lake flanked by group of soldiers' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'flanked by' means one lot of letters goes inside another (other letters go on other side).
'group of soldiers' becomes 'unit' (unit can mean a group of soldiers).
'l' placed inside 'unit' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Without illumination" , "Perhaps invisible" , "Not yet burning" , "With no lighting" , "dim" .)