Study upset dog? Let me think... (3,2)
I believe the answer is:
mug up
'study' is the definition.
(I know that mug up has the is a more specific form of the action study)
'upset dog? let me think' is the wordplay.
'upset' shows that the letters should be reversed in order.
'dog?' becomes 'pug' (pug is a kind of dog).
'let me think' becomes 'um' (expression of hesitation).
'pug'+'um'='pugum'
'pugum' written backwards gives 'MUG UP'.
(Other definitions for mug up that I've seen before include "Quickly revise" , "Learn quickly (for exam)" , "Revise for an exam" , "revise intensively" , "Cram" .)