Promise the Spanish small IOU? (6)
I believe the answer is:
vowels
'iou?' is the definition.
Although both the answer and definition are nouns, they are not in the same form of the noun.
'promise the spanish small' is the wordplay.
'promise' becomes 'vow' (I've seen this before).
'the spanish' becomes 'el' ('the' in Spanish).
'small' becomes 's' (abbreviation - e.g. clothes size).
'vow'+'el'+'s'='VOWELS'
Can you help me to learn more?
(Other definitions for vowels that I've seen before include "Speech sounds made with the vocal tract open" , "'Five letters, not consonants (6)'" , "Five speech sounds" , "aeiou" , "A e i o u" .)