Irritating smirk spreading with old energy (7)
I believe the answer is:
irksome
'irritating' is the definition.
(I've seen this before)
'smirk spreading with old energy' is the wordplay.
'spreading with' indicates anagramming the letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'energy' becomes 'e' (physics symbol as in E=mc2).
'smirk' mixed up with 'o' makes 'irksom'.
'irksom'+'e'='IRKSOME'
(Other definitions for irksome that I've seen before include "Annoying - tedious" , "Burdensome and tedious" , "'Tedious, tiresome (7)'" , "Smoker I find annoying" , "Troublesome, vexing" .)