It's after frisky fun that one's out of condition (5)
I believe the answer is:
unfit
'out of condition' is the definition.
(similar in meaning)
'it's after frisky fun that one's' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'after' says to put letters next to each other.
'frisky' indicates anagramming the letters.
'that one' becomes 'i' (Roman numeral).
'fun' with letters rearranged gives 'unf'.
'unf'+'i'='unfi'
't' put after 'unfi' is 'UNFIT'.
(Other definitions for unfit that I've seen before include "Not in good physical shape (5)" , "Inappropriate" , "Not healthy" , "Unsuitable" , "Incapable" .)