Fruitful following a diet, finally (3)
I believe the answer is:
fat
'fruitful' is the definition.
(similar in meaning)
'following a diet finally' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'finally' says to take the final letters.
The last letter of 'diet' is 't'.
'f'+'a'+'t'='FAT'
(Other definitions for fat that I've seen before include "Greasy substance" , "Substantial" , "and 24: Overpaid City type" , "Obese, having spare tyre etc" , "eg Lard" .)