Italian poet has very big day coming up (5)
I believe the answer is:
Here is my best explanation:
'italian poet' is the definition.
(I've seen this before)
'very big day coming up' is the subsidiary indication.
'very big' becomes 'os'.
'day' becomes 'sat'.
'coming up' says the letters should be written backwards.
'ossat' written backwards gives 'tasso'.
'has' is the link.
(Other definitions for tasso that I've seen before include "Poet", "Torquato -, nineteenth century Italian poet", "Torquato -, sixteenth century Italian poet", "Torquato --, Italian poet of the 16th-century" and "Torquato ---, It. poet".)