Old couple start tango off to get in shape (7)
I believe the answer is:
octagon
'shape' is the definition.
(octagon is a kind of shape)
'old couple start tango off to get' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'start' indicates taking the first letters.
'off' is an anagram indicator.
'to get' means one lot of letters goes inside another.
The initial letter of 'couple' is 'c'.
'tango' is an anagram of 'tagon'.
'c' placed inside 'tagon' is 'ctagon'.
'o'+'ctagon'='OCTAGON'
'in' is the link.
(Other definitions for octagon that I've seen before include "regular form" , "plane figure" , "Figure with 8 sides" , "Eight-sided figure" , "Eight sided figure" .)