Problem upset English -- it's deliberate (4)
I believe the answer is:
muse
'deliberate' is the definition.
(I've seen this in another clue)
'problem upset english' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'upset' shows that the letters should be reversed in order.
'english' becomes 'E' (abbreviation).
'sum' back-to-front is 'mus'.
'mus'+'e'='MUSE'
'it's' acts as a link.
(Other definitions for muse that I've seen before include "Ponder or meditate" , "Contemplation" , "Be absorbed in thought" , "Contemplate -- a goddess of the liberal arts" , "Ponder, say meditatively" .)