Old, are apt to stretch (6)
I believe the answer is:
extend
'stretch' is the definition.
('extend' can be a synonym of 'stretch')
'old are apt' is the wordplay.
'old' becomes 'ex'.
'are apt' becomes 'tend'.
'ex'+'tend'='EXTEND'
'to' acts as a link.
(Other definitions for extend that I've seen before include "Enlarge, widen" , "Reach out, say the hand of friendship" , "Stretch out - enlarge" , "Straighten out" , "Carry on" .)