Trade union splits very good sum (3,2)
I believe the answer is:
tot up
'sum' is the definition.
(I know that sum can be written as tot up)
'trade union splits very good' is the wordplay.
I cannot quite understand how this works, but
'union' could be 'tu' (short for trade union) and 'tu' is found in the answer.
The remaining letters 'top' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tot up that I've seen before include "Add (some numbers)" , "Determine the sum of" , "Summate" , "Do summer job" , "Sum - total" .)