A pair of characters in lower hot pants (9)
I believe the answer is:
diphthong
'a pair of characters' is the definition.
(two letters representing a particular vowel sound)
'lower hot pants' is the wordplay.
'lower' becomes 'dip' (dipping is a kind of lowering).
'hot' becomes 'h' (abbreviation eg on taps).
'pants' becomes 'thong' (type of underwear).
'dip'+'h'+'thong'='DIPHTHONG'
'in' acts as a link.
(Other definitions for diphthong that I've seen before include "Two-vowel sound" , "Sound of two vowels in one syllable" , "Two vowels pronounced as one syllable" , "Combined vowel" , "Combination of two vowels" .)