Rat out in spring after first sign of sun (4)
I believe the answer is:
shop
'rat' is the definition.
(both can mean to betray someone)
'in spring after first sign of sun' is the wordplay.
'in spring' becomes 'hop' (hopping is a kind of springing. I am not sure about the 'in' bit.).
'after' is a charade indicator (letters next to each other).
'first sign of' suggests taking the first letters.
The initial letter of 'sun' is 's'.
'hop' after 's' is 'SHOP'.
'out' is the link.
Can you help me to learn more?
(Other definitions for shop that I've seen before include "Store - grass up" , "Place to buy" , "Store, outlet" , "Be disloyal to" , "Retail outlet" .)