The best swot welcomes start of exams (5)
I believe the answer is:
cream
'the best' is the definition.
(I've seen this before)
'swot welcomes start of exams' is the wordplay.
'swot' becomes 'cram' ('cram' can be a synonym of 'swot').
'welcomes' is an insertion indicator (in sense of 'invites in').
'start of' suggests taking the first letters.
The first letter of 'exams' is 'e'.
'cram' placed around 'e' is 'CREAM'.
(Other definitions for cream that I've seen before include "Best - ointment" , "It rises to the top, the best" , "Beat" , "Very best; pale yellow" , "best people" .)