I start to loosen in time, becoming nimble (5)
I believe the answer is:
agile
'nimble' is the definition.
(agile can mean nimble or athletic)
'i start to loosen in time' is the wordplay.
'start to' says to take the initial letters.
'in' means one lot of letters goes inside another.
'time' becomes 'age' (an age is a period of time).
The first letter of 'loosen' is 'l'.
'i'+'l'='il'
'il' going within 'age' is 'AGILE'.
'becoming' acts as a link.
(Other definitions for agile that I've seen before include "Lithely moving" , "Limber" , "Sprightly" , "Quick and easy-moving" , "Able to move nimbly" .)