Watch row about source of money (5)
I believe the answer is:
timer
'watch' is the definition.
(thesaurus)
'row about source of money' is the wordplay.
'row' becomes 'tier' (tier is a kind of row**).
'about' indicates putting letters inside.
'source of' indicates taking the first letters.
The initial letter of 'money' is 'm'.
'tier' enclosing 'm' is 'TIMER'.
(Other definitions for timer that I've seen before include "Eg, clock" , "It measures intervals" , "Eg, stopwatch" , "Clocklike device" , "One may be alarmed" .)