Line among pages may, unfortunately, mess around with one's feelings (4,5)
I believe the answer is:
play games
'mess around with one's feelings' is the definition.
(as in playing games with someone)
'line among pages may unfortunately' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'among' indicates putting letters inside.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'pages'+'may'='pagesmay'
'pagesmay' with letters rearranged gives 'paygames'.
'l' placed within 'paygames' is 'PLAY GAMES'.
(Another definition for play games that I've seen is " Fool around".)