Trees discovered in the Spanish manuscript (4)
I believe the answer is:
elms
'trees' is the definition.
(elm is a kind of tree)
'the spanish manuscript' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'manuscript' becomes 'ms' (abbreviation for manuscript).
'el'+'ms'='ELMS'
'discovered in' acts as a link.
(Other definitions for elms that I've seen before include "rugged feature of country churchyard" , "Deciduous trees with rough serrated leaves" , "Trees sound like Cockney steering devices" , "Trees (of late, prone to disease)" , "may have many leaves" .)