Perhaps a solemn promise by the Spanish (5)
I believe the answer is:
vowel
'perhaps a solemn' is the definition.
I can't tell whether this defines the answer.
'promise by the spanish' is the wordplay.
'promise' becomes 'vow' (I've seen this in another clue).
'by' says to put letters next to each other.
'the spanish' becomes 'el' ('the' in Spanish).
'vow'+'el'='VOWEL'
Can you help me to learn more?
(Other definitions for vowel that I've seen before include "Open sound in speech" , "Possibly a" , "Not a consonant" , "aeio or u" , "Kind of letter of the alphabet" .)