Ring engineer up in terminus (6)

'ring' is the definition.
(thesaurus)

'engineer up in terminus' is the wordplay.
'engineer' becomes 'rig' (as a verb).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates putting letters inside.
'terminus' becomes 'end' (eg the end of a train line is its terminus).
'rig' in reverse letter order is 'gir'.
'gir' placed within 'end' is 'ENGIRD'.