Sit Reg up with the cats
I believe the answer is:
tigers
'cats' is the definition.
(I know this)
'sit reg up' is the wordplay.
'up' is an anagram indicator (letters get chucked up).
'sit'+'reg'='sitreg'
'sitreg' anagrammed gives 'TIGERS'.
'with the' acts as a link.
(Other definitions for tigers that I've seen before include "Large felines of forests" , "Large forest felines" , "Big striped cats poetically burning bright" , "Sir, get to the animals" , "Large striped felines" .)