Heath's in charge during long period
I believe the answer is:
erica
'heath's' is the definition.
(erica is a kind of heath)
'in charge during long period' is the wordplay.
'in charge' becomes 'ic' (abbreviation for 'in charge').
'during' is an insertion indicator.
'long period' becomes 'era' (I've seen this before).
'ic' inserted into 'era' is 'ERICA'.
(Other definitions for erica that I've seen before include "Plant genus of garden heather" , "I care about Heather, oddly" , "Woman" , "found on the moors" , "I care (anag) - plant - girl" .)