Work too hard, getting cross after obvious answer (7)
I believe the answer is:
overtax
'work too hard' is the definition.
(to overtax someone is to work them too hard)
'cross after obvious answer' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'after' is a charade indicator (letters next to each other).
'obvious' becomes 'overt' (similar in meaning).
'answer' becomes 'a' (abbreviation eg in Q and A).
'overt'+'a'='overta'
'x' after 'overta' is 'OVERTAX'.
'getting' acts as a link.
(Other definitions for overtax that I've seen before include "Impose too much strain on" , "Press too hard" , "Place too great a burden on, as Revenue officials do" , "Require too much" , "Make excessive demands on (a person's abilities)" .)