Slow to catch rook in military loo (7)
I believe the answer is:
latrine
'military loo' is the definition.
(I've seen this before)
'slow to catch rook in' is the wordplay.
'slow' becomes 'late' (I've seen this in another clue).
'to catch' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'r'+'in'='rin'
'late' enclosing 'rin' is 'LATRINE'.
(Other definitions for latrine that I've seen before include "Army lavatory" , "where to go in the army" , "Camp toilet" , "A military lavatory" , "public toilet" .)