Return help and gauge line (8)
I believe the answer is:
diameter
'line' is the definition.
(diameter is a kind of line)
'return help and gauge' is the wordplay.
'return' is a reversal indicator.
'help' becomes 'aid' (synonyms).
'and' means one lot of letters go next to another.
'gauge' becomes 'meter' (I've seen this in another clue).
'aid' back-to-front is 'dia'.
'dia'+'meter'='DIAMETER'
(Other definitions for diameter that I've seen before include "Straight line drawn through centre of circle (8)" , "Circle's width" , "width of bar" , "It crosses the circle at its widest" , "Line connecting the centre of a circle with two points on the perimeter" .)