Swear at the Spanish in a letter (5)
I believe the answer is:
vowel
'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' says to put letters next to each other.
'the spanish' becomes 'el' ('the' in Spanish).
'vow'+'el'='VOWEL'
'in a' is the 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" .)