Slow on the uptake - kind of angle (6)
I believe the answer is:
obtuse
I believe this is a double definition.
'slow on the uptake' is the first definition.
(I've seen this in another clue)
'kind of angle' is the second definition.
(I've seen this in another clue)
(Other definitions for obtuse that I've seen before include "Slow, dull" , "Not very perceptive or sensitive" , "'Dense, slow (6)'" , "Dull or blunt" , "Insensitive - more than a right angle" .)