Popular supporter extremely cheeky at the beginning (7)
I believe the answer is:
infancy
'the beginning' is the definition.
(I've seen this before)
'popular supporter extremely cheeky' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'supporter' becomes 'fan' (fan is a kind of supporter).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'cheeky' with its centre removed is 'cy'.
'in'+'fan'+'cy'='INFANCY'
'at' is the link.
(Other definitions for infancy that I've seen before include "minority" , "Early stage of development" , "First stage of life" , "beginning" , "Early years" .)