Reckon to keep in shape (6)
I believe the answer is:
figure
This could be a double definition.
'reckon' is the first definition.
(synonyms)
'keep in shape' is the second definition.
'figure' can be an answer for 'shape' (synonyms). I am not certain of the 'keep in' bit.
'to' is the link.
Can you help me to learn more?
(Other definitions for figure that I've seen before include "Work out" , "Diagram or digit" , "Shape; amount of money" , "Shape; number" , "The written symbol of a number" .)