Ease, lazily at first, into a set (5)
I believe the answer is:
allay
'ease' is the definition.
(synonyms)
'lazily at first into a set' is the wordplay.
'at first' suggests taking the first letters.
'into' means one lot of letters goes inside another.
'set' becomes 'lay' (laying is a kind of setting).
The first letter of 'lazily' is 'l'.
'a'+'lay'='alay'
'l' inserted into 'alay' is 'ALLAY'.
(Other definitions for allay that I've seen before include "Mitigate" , "Calm fears" , "Assuage, ease" , "Relieve, soothe" , "Calm, put at rest" .)