Wound up the Spanish firm (6)
I believe the answer is:
stable
'firm' is the definition.
('stable' can be similar in meaning to 'firm')
'wound up the spanish' is the wordplay.
'wound' becomes 'stab' (I've seen this before**).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'the spanish' becomes 'el'.
'el' reversed gives 'le'.
'stab'+'le'='STABLE'
(Other definitions for stable that I've seen before include "Not likely to fail" , "Durable; horse shed" , "Bleats in a horse-house" , "Steady, enduring" , "Sound" .)