Sometimes I now ache, line dancing (4,2,1,5)
I believe the answer is:
once in a while
'sometimes' is the definition.
(I know that sometimes can be written as once in a while)
'i now ache line dancing' is the wordplay.
'dancing' indicates anagramming the letters.
'i'+'now'+'ache'+'line'='inowacheline'
'inowacheline' anagrammed gives 'ONCE IN A WHILE'.
(Other definitions for once in a while that I've seen before include "Every now and again" , "Not very often" , "At times" , "Occasionally" .)