Friend at length to become less interesting (4)
I believe the answer is:
pall
'become less interesting' is the definition.
(I've seen this before)
'friend at length' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend).
'at' is a charade indicator (letters next to each other).
'length' becomes 'l'.
'pal'+'l'='PALL'
'to' acts as a link.
(Other definitions for pall that I've seen before include "Become boring and uninteresting" , "tedious here" , "Become tiresome" , "Cloth over a coffin - become insipid" , "Become less appealing" .)