Discretion when trouble starts at the tractor centre (4)
I believe the answer is:
tact
'discretion' is the definition.
(tactful can mean discreet)
'trouble starts at the tractor centre' is the wordplay.
'starts' suggests taking the first letters.
'at' is a charade indicator (letters next to each other).
'centre' says to take the centre.
The centre of 'tractor' is 'act'.
The first letter of 'trouble' is 't'.
't'+'act'='TACT'
'when' acts as a link.
(Other definitions for tact that I've seen before include "Sensitivity in dealing with people" , "Consideration in dealing with others" , "Understanding" , "measure of diplomacy?" , "Diplomatic skill" .)