Sarah to become fatigued and cross (7)
I believe the answer is:
Here is my best explanation:
'cross' is the definition.
(saltire is a kind of cross)
'sarah to become fatigued' is the wordplay.
'sarah' becomes 'sal' (Sally is a common pet form of Sarah).
'to become fatigued' becomes 'tire' (I've seen this before).
'and' acts as a link.