Sit learner at back of form (5)
I believe the answer is:
model
'sit' is the definition.
(synonyms)
'learner at back of form' is the wordplay.
'learner' becomes 'l'.
'at back of' is a charade indicator (letters next to each other).
'form' becomes 'mode' ().
'l' put after 'mode' is 'MODEL'.
(Other definitions for model that I've seen before include "Exemplary - mannequin" , "Poser for painter or photographer" , "Artist's sitter" , "Miniature replica" , "She parades on the catwalk" .)