They harvest about one every second (7)
I believe the answer is:
reapers
'they harvest' is the definition.
Although both the answer and 'harvest' are nouns, they are not in the same form of the noun.
'about one every second' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'one every' becomes 'per' ('per' means 'for each' or 'for every').
'second' becomes 's' (common abbreviation).
'rea'+'per'+'s'='REAPERS'
Can you help me to learn more?
(Other definitions for reapers that I've seen before include "More than one cropper" , "one could be grim!" , "Harvesters" , "Gatherers" .)