The Spanish learner's measure (3)
I believe the answer is:
ell
'measure' is the definition.
(historical unit of length)
'the spanish learner's' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'el'+'l'='ELL'
(Other definitions for ell that I've seen before include "Old textile measure" , "Old 45-inch measure" , "approximately 45 inches" , "Former textile measure" , "Old unit of length" .)