Mundane blunder in batch overturned experiment (11)
I believe the answer is:
'mundane' is the definition.
'blunder in batch overturned experiment' is the wordplay.
'blunder' becomes 'err' (I've seen this before).
'in' means one lot of letters goes inside another.
'batch' becomes 'set'.
'overturned' says the letters should be written backwards.
'experiment' becomes 'trial' (trial is a kind of experiment).
'err' going inside 'set' is 'serret'.
'serret' back-to-front is 'terres'.
(Other definitions for terrestrial that I've seen before include "Relating to dry land" , "Relating to the earth" , "Land-based" , "Living on the ground" , "Not aquatic, arboreal, or aerial" .)