2d ends with ...USES (after the 'discovered'/uncovered word) a medical plural.
10d becomes birds after the thematic addition (the waste from threshing)is added to the front of the 6 word answer (def 'measures').
13d The 4 letter removal is a type of bed (4 letters) but also a theme word - the wordplay for the full answer is 'first timer' = DEB + 'discarded' (6 letters) minus the initial 'j'.
Chaff doesn't come from anywhere (this answer is one of the thematic 10)...it's the thematic bit that has to be added to that answer
similarly with bunk ...it just comes out ...it doesn't go anywhere
Sorry, hankir - the thematic ins + outs don't work like that, it's sufficient that that the 'G...' (in general) is put in, or taken out of the individual answers.
There are a couple of 'swapped' pairs, ('R...' and 'L...') but most are singletons - 'B...' 'C...' 'G...' 'M...' + 'R...'
I think it would have been a very limiting constraint, otherwise (and I very much enjoyed the puzzle as it is).