Addition to will makes Spanish hero retreat in wind (7)
I believe the answer is:
codicil
'addition to will' is the definition.
(I've seen this before)
'hero retreat in wind' is the wordplay.
'hero' becomes 'cid' (El Cid, Spanish military hero).
'retreat' is a reversal indicator.
'in' means one lot of letters goes inside another.
'wind' becomes 'coil' (coiling is a kind of winding).
'cid' back-to-front is 'dic'.
'dic' going inside 'coil' is 'CODICIL'.
'makes spanish' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Addition to a will" , "Modifier to will" , "Appendix to a will" , "Will's rider" , "part of will" .)