Before I file this away, can I get some help parsing this, specifically the origin of the extra letter...
21d - Flavour our Gallic friend bolsters using mushrooms initially (5)
As far as I can tell, the first two letters of the answer come from 'using mushrooms initially' and the final three from 'Gallic friend' with the definition being' 'flavour'. That just leaves 'our', but I don't see how changing that to 'or' improves the wordplay or even alters it - where have I gone wrong?