The old strenuously stay zestful (6)
I believe the answer is:
yeasty
'zestful' is the definition.
(synonyms)
'the old strenuously stay' is the wordplay.
I cannot quite see how this works, but
an anagram of 'stay' is 'asty' which is located in the answer.
'the old' could be 'ye' and 'ye' is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for yeasty that I've seen before include "having ability to rise?" , "Frothy - insubstantial" , "upheaval" , "Like a fermenting agent" , "Like taste of home brew" .)