Tetchy Gershwin's upsetting the Spanish writer (9)
I believe the answer is:
irascible
'tetchy' is the definition.
(I've seen this before)
'gershwin's upsetting the spanish writer' is the wordplay.
'gershwin's' becomes 'ira's' (lyricist Ira Gershwin, brother of George).
'upsetting' says the letters should be written in reverse.
'the spanish' becomes 'el' ('the' in Spanish).
'writer' becomes 'bic' (kind of pen).
'el'+'bic'='elbic'
'elbic' reversed gives 'cible'.
'iras'+'cible'='IRASCIBLE'
(Other definitions for irascible that I've seen before include "Quick to anger" , "Easily aroused to anger" , "peppery" , "testy" , "Short-tempered" .)