Work unit in Chester garden (3)
I believe the answer is:
erg
'work unit' is the definition.
(erg is a kind of work unit)
'in chester garden' is the wordplay.
'in' indicates a hidden word.
'ERG' can be found hidden inside 'chester garden'.
(Other definitions for erg that I've seen before include "What's used to record work" , "Energy expended" , "Amount of work" , "Shifting dunes in Sahara" , "Unit of work or energy" .)