Tended to run up spend, oddly (6)
I believe the answer is:
nursed
'tended to' is the definition.
(I've seen this in another clue)
'run up spend oddly' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'oddly' indicates alternate letters (letters in odd-numbered positions).
The alternating letters of 'spend' are 'sed'.
'run' written backwards gives 'nur'.
'nur'+'sed'='NURSED'
(Other definitions for nursed that I've seen before include "Tended to while ill" , "given medical attention" , "Tended to the sick, cared for" , "26dn the ill" , "Took care of while ill" .)