Stain made by learner in screw up (6)
I believe the answer is:
blotch
'stain' is the definition.
(I have seen 'Large stain ' mean 'blotch' so perhaps 'stain' could also mean 'blotch')
'learner in screw up' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'screw up' becomes 'botch' (synonyms).
'l' placed into 'botch' is 'BLOTCH'.
'made by' is the link.
(Other definitions for blotch that I've seen before include "Spot of irregular discoloration" , "Unsightly patch" , "Large stain" , "Irregular patch or mark" , "Irregular coloured mark" .)