Group getting cold in tub (5)
I believe the answer is:
batch
'group' is the definition.
(batch is a kind of group)
'cold in tub' is the wordplay.
'cold' becomes 'c' (eg on taps).
'in' means one lot of letters goes inside another.
'tub' becomes 'bath' ('bath' can be a synonym of 'tub').
'c' placed inside 'bath' is 'BATCH'.
'getting' acts as a link.
(Other definitions for batch that I've seen before include "Group of similar people" , "All the bread produced at one baking" , "Quantity of goods produced" , "Group of items (as handled)" , "Collection of items handled together" .)