Time of year bound to unite swallows beginning to migrate (10)
I believe the answer is:
springtime
'time of year' is the definition.
(I know that springtime is a type of time of year)
'bound to unite swallows beginning to migrate' is the wordplay.
'bound' becomes 'spring' (both can mean to leap).
'to' means one lot of letters go next to another.
'unite' becomes 'tie' (both can mean to join).
'swallows' indicates putting letters inside.
'beginning to' says to take the initial letters.
The initial letter of 'migrate' is 'm'.
'tie' placed around 'm' is 'time'.
'spring'+'time'='SPRINGTIME'
(Other definitions for springtime that I've seen before include "Season loved by lovers" , "era of promise" , "When trees are budding" , "Growing season" .)