Constant horseplay oldies partially backed (5)
I believe the answer is:
loyal
'constant' is the definition.
(thesaurus)
'horseplay oldies partially backed' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'backed' says the letters should be written in reverse.
'layol' can be found hidden inside 'horseplay oldies'.
'layol' written backwards gives 'LOYAL'.
(Other definitions for loyal that I've seen before include "True to one's country etc" , "Unwaveringly devoted to friend or cause" , "Faithful to country, cause or friend" , "Dependable" , "True-hearted" .)