Is feeling cold in small area of activity by river, sunless initially (7)

'is feeling cold' is the definition.
I can't tell whether this defines the answer.

'small area of activity by river sunless initially' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'area of activity' becomes 'hive' (I can't explain this - if you can you should believe this answer much more).
'by' means one lot of letters go next to another.
'initially' indicates taking the first letters.
The initial letters of 'river sunless' is 'rs'.
's'+'hive'+'rs'='SHIVERS'

'in' is the link.

Can you help me to learn more?