'Earthly time slips' describing Einstein's first experiment (11)
I believe the answer is:
terrestrial
'earthly' is the definition.
('terrestrial' can be a synonym of 'earthly')
'time slips describing einstein's first experiment' is the wordplay.
'time' becomes 't'.
'slips' becomes 'errs' ('err' can be a synonym of 'slip'**).
'describing' is an insertion indicator (describe can mean to form the outline of).
'first' says to take the initial letters.
'experiment' becomes 'trial' (trial is a kind of experiment).
The first letter of 'einsteins' is 'e'.
'errs' going around 'e' is 'erres'.
't'+'erres'+'trial'='TERRESTRIAL'
(Other definitions for terrestrial that I've seen before include "Not aquatic, arboreal, or aerial" , "Land-based" , "Relating to the earth" , "Living on the ground" , "Relating to dry land" .)