Taking exercise speedily produces returns (6)
I believe the answer is:
'returns' is the definition.
('yield' can be a synonym of 'return')
'taking exercise speedily produces' is the wordplay.
'taking' suggests deleting specific letters.
'exercise' becomes 'pe' (Physical Education).
'produces' indicates an anagram.
'speedily' with 'pe' taken out is 'sedily'.
'sedily' anagrammed gives 'YIELDS'.
(Other definitions for yields that I've seen before include "Amounts harvested" , "bows" , "Surrenders like harvests" , "financial returns" , "Surrenders the crops" .)