Reckon branch of mathematics had us lost (9)
I believe the answer is:
calculate
'reckon' is the definition.
(synonyms)
'branch of mathematics had us lost' is the wordplay.
'branch of mathematics' becomes 'calculus' (I've seen this before).
'had' becomes 'ate' (e.g. I had an ice cream).
'lost' suggests deleting specific letters.
'calculus'+'ate'='calculusate'
'calculusate' with 'us' taken away is 'CALCULATE'.
(Other definitions for calculate that I've seen before include "Work out mathematically" , "Compute or reckon" , "Determine mathematically" , "Solve by mathematical procedure" , "'Work out, compute (9)'" .)