Suspect everybody returned during term as planned (5,1,3)
I believe the answer is:
smell a rat
'suspect' is the definition.
(I've seen this before)
'everybody returned during term as planned' is the wordplay.
'everybody' becomes 'all' (I've seen this before).
'returned' says the letters should be written in reverse.
'during' is an insertion indicator.
'planned' indicates anagramming the letters.
'all' written backwards gives 'lla'.
'term'+'as'='termas'
'termas' anagrammed gives 'smerat'.
'lla' inserted within 'smerat' is 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Suspect deception" , "Detect something suspicious" , "Find something fishy" , "Suspect something's amiss" , "Have suspicion something is afoot" .)