Tally up and take in less (5)
I believe the answer is:
minus
'less' is the definition.
(I've seen this in other clues)
'tally up and take in' is the wordplay.
'tally' becomes 'sum' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'and take' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'sum' reversed gives 'mus'.
'mus' enclosing 'in' is 'MINUS'.
Can you help me to learn more?
(Other definitions for minus that I've seen before include "(Of temperature) below zero" , "Negative arithmetic operation" , "Take-away (sign)" , "Disadvantage" , "'Take away' sign" .)