Inform of allergic reaction to tin? (6)
I believe the answer is:
snitch
'inform' is the definition.
(I've seen this before)
'allergic reaction to tin?' is the wordplay.
'allergic reaction' becomes 'itch' (I can't explain this - if you can you should believe this answer much more).
'to' means one lot of letters go next to another (I've seen this in other clues).
'tin?' becomes 'sn' ('Sn' can be a synonym of 'tin').
'itch' after 'sn' is 'SNITCH'.
'of' acts as a link.
(Other definitions for snitch that I've seen before include "Grass" , "I'm telling!" , "Telltale (sl.)" , "Inform on, tell tales" , "Steal; inform (on someone)" .)