In old age, starts to stretch out at full length (2,7)
I believe the answer is:
in extenso
'at full length' is the definition.
(I've seen this in another clue)
'in old age starts to stretch out' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'age' becomes 'ten' (ten years old is an age).
'starts to' says to take the initial letters.
The first letters of 'stretch out' is 'so'.
'in'+'ex'+'ten'+'so'='IN EXTENSO'
(Other definitions for in extenso that I've seen before include "In full (Latin)" , "for as long as possible?" .)