Liability set to return in experimental environment (4,3)
I believe the answer is:
test bed
'experimental environment' is the definition.
'liability set to return' is the wordplay.
'liability' becomes 'debt' (debt is a kind of liability).
'set to' becomes 'set' (synonyms).
'return' says the letters should be written backwards.
'debt'+'set'='debtset'
'debtset' reversed gives 'TEST-BED'.
'in' is the link.
(Other definitions for test bed that I've seen before include "engineers are working on it" , "Practise sleeping" , "place for experimental trials" , "trial equipment" .)