Untrustworthy? Then do field trips (2,3,6)
I believe the answer is:
on the fiddle
'untrustworthy?' is the definition.
(someone on the fiddle is fraudulent or cheating)
'then do field trips' is the wordplay.
'trips' indicates anagramming the letters.
'then'+'do'+'field'='thendofield'
'thendofield' anagrammed gives 'ON THE FIDDLE'.
(Other definitions for on the fiddle that I've seen before include "Dishonest" , "Running a racket" , "Engaged in cheating" , "leader of the orchestra?" , "Cheating (colloq.)" .)