Military officer guarding extra-large stray (4)
I believe the answer is:
lost
'stray' is the definition.
(thesaurus)
'military officer guarding extra-large' is the wordplay.
'military officer' becomes 'lt' (lieutenant).
'guarding' indicates putting letters inside (inserted letters are protected or guarded).
'extra large' becomes 'os' (Over Size - library term for books).
'lt' going around 'os' is 'LOST'.
(Other definitions for lost that I've seen before include "See 8" , "Unable to be found" , "Unable to find the way forward" , "Missing" , "Mogul empire official; rich person" .)