Learner welcomed by second chess player (5)
I believe the answer is:
black
'chess player' is the definition.
(I've seen this before)
'learner welcomed by second' is the wordplay.
'learner' becomes 'l'.
'welcomed by' is an insertion indicator (I've seen this in other clues).
'second' becomes 'back' (to second a motion is to back it).
'l' going inside 'back' is 'BLACK'.
(Other definitions for black that I've seen before include "Chess player" , "Unofficial" , "Colour of soot" , "Colour of coal" , "Desperate" .)