Always enter to work out with a friend (9)
I believe the answer is:
eternally
'always' is the definition.
(thesaurus)
'enter to work out with a friend' is the wordplay.
'to work out' is an anagram indicator.
'with' says to put letters next to each other.
'a friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'enter' with letters rearranged gives 'etern'.
'etern'+'ally'='ETERNALLY'
(Other definitions for eternally that I've seen before include "Always and forever" , "Always and ever" , "Nearly let out forever" , "constantly" , "For evermore" .)