Beds used by active working girls when retiring (6)
I believe the answer is:
strata
'beds' is the definition.
(I have seen 'hard beds' mean 'strata' so perhaps 'beds' could also mean 'strata')
'by active working girls when retiring' is the wordplay.
'by' means one lot of letters go next to another.
'active' becomes 'a' (grammatical abbreviation).
'working girls' becomes 'tarts' ('tart' can be a synonym of 'working girl').
'when retiring' shows that the letters should be reversed in order (retire can mean to go back).
'tarts' backwards is 'strat'.
'a' after 'strat' is 'STRATA'.
'used' is the link.
(Other definitions for strata that I've seen before include "Beds of sedimentary rock" , "Layers of rock on earth's surface" , "Layers or thicknesses of earth" , "Layers as of rocks in earth's crust" , "Geological levels" .)