Missing lieutenant takes in old soldiers initially (4)
I believe the answer is:
lost
'missing' is the definition.
(similar in meaning)
'lieutenant takes in old soldiers initially' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'takes in' is an insertion indicator.
'initially' indicates taking the first letters.
The initial letters of 'old soldiers' is 'os'.
'lt' placed around 'os' is 'LOST'.
(Other definitions for lost that I've seen before include "Missing" , "Not recognising one's whereabouts" , "Mogul empire official; rich person" , "TV series!" , "Not having known where one is" .)