He uses Lyons tea, I've heard, behind their back (8)

'back' is the definition.
Both the answer and definition are adverbs.
Perhaps they are linked in a way I don't understand?

'he uses lyons tea i've heard behind' is the wordplay.
I cannot really see how this works, but
'he' could be 'sly' and 'sly' is found in the answer.
'tea' could be 't' (I've seen this in other clues) and 't' is present in the answer.
This explanation may well be incorrect...

'their' acts as a link.

Can you help me to learn more?