Having tin, long for informer (6)
I believe the answer is:
snitch
'for informer' is the definition.
(snitch is a kind of informer)
'having tin long' is the wordplay.
'having tin' becomes 'sn' (I can't explain this - if you can you should give a lot more credence to this answer).
'long' becomes 'itch' (to long or itch for something is to desire it).
'sn'+'itch'='SNITCH'
Can you help me to learn more?
(Other definitions for snitch that I've seen before include "who tells on his classsmate?" , "Inform on someone in slangy way" , "Inform on, tell tales" , "Telltale (sl.)" , "Steal; inform (on someone)" .)