Lying boss almost in late (9)
I believe the answer is:
recumbent
'lying' is the definition.
(I've seen this before)
'boss almost in late' is the wordplay.
'boss' becomes 'umbo' (the boss on a shield).
'almost' means to remove the last letter (most of the word but not all of it).
'in' means one lot of letters goes inside another.
'late' becomes 'recent' (similar in meaning).
'umbo' with its final letter taken away is 'umb'.
'umb' placed inside 'recent' is 'RECUMBENT'.
(Other definitions for recumbent that I've seen before include "flat?" , "Lying down comfortably" .)