Swear at the Spanish in a letter (5)
I believe the answer is:
vowel
'a letter' is the definition.
(vowel is a kind of letter)
'swear at the spanish' is the wordplay.
'swear' becomes 'vow' (I've seen this before).
'at' is a charade indicator (letters next to each other).
'the spanish' becomes 'el'.
'vow'+'el'='VOWEL'
'in' acts as a link.
(Other definitions for vowel that I've seen before include "A or e, say" , "Letter such as i or u" , "Not a consonant" , "Letter representing sound" , "aeio or u" .)