Chess player losing learner’s support (4)
I believe the answer is:
back
'support' is the definition.
(to back is to support or endorse)
'chess player losing learner's' is the wordplay.
'chess player' becomes 'black' (one of the two players in a game of chess).
'losing' is a deletion indicator.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'black' with 'l' removed is 'BACK'.
(Other definitions for back that I've seen before include "Series of popular time-travel movies" , "Support (from behind?)" , "Support - football player" , "part of the body" , "Support a play - player" .)