Source of milk runt initially guzzled — a pleasant surprise (5)
I believe the answer is:
treat
'a pleasant surprise' is the definition.
(a treat is a pleasant gift or favour)
'source of milk runt initially guzzled' is the wordplay.
'source of milk' becomes 'teat' (body part which dispenses milk).
'initially' indicates taking the first letters.
'guzzled' indicates putting letters inside (inserted letters get guzzled up).
The first letter of 'runt' is 'r'.
'teat' enclosing 'r' is 'TREAT'.
(Other definitions for treat that I've seen before include "Enjoyable surprise gift" , "Source of gratification" , "Try to heal" , "Shout" , "Pay for someone's food or drink or look after them medically" .)