The whole time a lot developed (5)
I believe the answer is:
total
'the whole' is the definition.
(total is a kind of whole)
'time a lot developed' is the wordplay.
'time' becomes 't'.
'developed' indicates anagramming the letters.
'a'+'lot'='alot'
'alot' is an anagram of 'otal'.
't'+'otal'='TOTAL'
(Other definitions for total that I've seen before include "Final tally" , "Come to" , "The whole, sum" , "Aggregate amount" , "Sum, aggregation" .)