Soon perspire regularly in full-body garment (6)
I believe the answer is:
onesie
'full-body garment' is the definition.
(type of one-piece garment)
'soon perspire regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'soon'+'perspire'='soonperspire'
The alternating letters of 'soonperspire' are 'ONESIE'.
'in' acts as a link.
(Other definitions for onesie that I've seen before include "One-piece full-body garment" , "Single-piece full body garment" .)