Don unknown old pretty clothes (7)
I believe the answer is:
quixote
'don' is the definition.
(Don Quixote, Spanish novel)
'unknown old pretty clothes' is the wordplay.
'unknown' becomes 'x' (algebra).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pretty' becomes 'quite' ('quite' can be a synonym of 'pretty'**).
'clothes' means one lot of letters goes inside another.
'x'+'o'='xo'
'xo' placed into 'quite' is 'QUIXOTE'.
(Other definitions for quixote that I've seen before include "Cervantes' hero" , "Foolish gentleman" , "Cervantes great creation Don . . . . . . ., man of La Mancha" , "Cervantes's Don" , "1 across --, knight-errant" .)