Hidden motive (5)
'hidden motive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for angle that I've seen before include "Glean (anag.)" , "Gradient" , "may be right" , "Standpoint" , "A way of approaching" .)
(Other definitions for ulterior that I've seen before include "Hidden or undisclosed, as motive may be" , "Not obvious or admitted - say motive" , "(Of a motive) beyond what is stated" , "Beyond what is revealed or evident" , "On the further side" .)