Italian poet has very big day coming up (5)
I believe the answer is:
tasso
'italian poet' is the definition.
(I've seen this before)
'very big day coming up' is the wordplay.
'very big' becomes 'os' (abbreviation for outsize).
'day' becomes 'sat' (I've seen this before).
'coming up' says the letters should be written in reverse.
'os'+'sat'='ossat'
'ossat' in reverse letter order is 'TASSO'.
'has' is the link.
(Other definitions for tasso that I've seen before include "Torquato -, sixteenth century Italian poet" , "Torquato --, Italian poet of the 16th-century" , "Torquato -, nineteenth century Italian poet" .)