Fat starts to appear newly ubiquitous between gym and work (6,3)
I believe the answer is:
peanut oil
'fat' is the definition.
(peanut oil is a kind of fat)
'starts to appear newly ubiquitous between gym and work' is the wordplay.
'starts to' suggests taking the first letters.
'between' is an insertion indicator.
'gym' becomes 'pe' (short for Physical Education).
'and' says to put letters next to each other.
'work' becomes 'toil' (toil is a kind of work).
The initial letters of 'appear newly ubiquitous' is 'anu'.
'pe'+'toil'='petoil'
'anu' going into 'petoil' is 'PEANUT OIL'.
(Another definition for peanut oil that I've seen is " substance inside soap?".)