Has got into exercise but it probably won't last (5)
I believe the answer is:
phase
'it probably won't last' is the definition.
The definition suggests a singular noun which matches the answer.
'has got into exercise' is the wordplay.
'has got' becomes 'has' ('have' can be a synonym of 'have got').
'into' indicates putting letters inside.
'exercise' becomes 'pe' (Physical Education).
'has' going inside 'pe' is 'PHASE'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for phase that I've seen before include "A particular point in the time of a cycle" , "Any distinct time period" , "Aspect of moon" , "Truman -, US novelist" , "stage in process of development" .)