Odd characters leaving real ale measure (3)
I believe the answer is:
ell
'measure' is the definition.
(historical unit of length)
'odd characters leaving real ale' is the wordplay.
'odd characters leaving' means one should take alternating letters (letters in odd-numbered positions removed).
'real'+'ale'='realale'
The alternating letters of 'realale' are 'ELL'.
(Other definitions for ell that I've seen before include "Embellish with old measure of length" , "Old textile measure" , "approximately 45 inches" , "Once, forty-five inches" , "Old unit of length" .)