Trunks used to be keeping one's backside in sight (6)
I believe the answer is:
waists
'trunks' is the definition.
(the waist is part of a person's trunk)
'used to be keeping one's backside in sight' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'keeping' is an insertion indicator.
'one's' becomes 'I's' (Roman numeral).
'backside in' says to take the final letters (the letter at the back).
The final letter of 'sight' is 't'.
'is'+'t'='ist'
'was' going around 'ist' is 'WAISTS'.
(Other definitions for waists that I've seen before include "Parts of bodies" , "blouses for Miss Obama" , "They come between the ribs and hips" .)