Muse on coming up with tattoos every so often (5)
I believe the answer is:
erato
'muse' is the definition.
(I know that Erato is a muse)
'on coming up with tattoos every so often' is the wordplay.
'on' becomes 're' (regarding).
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'with' is a charade indicator (letters next to each other).
'every so often' means one should take alternating letters.
The alternate letters of 'tattoos' are 'ato'.
're' in reverse letter order is 'er'.
'er'+'ato'='ERATO'
(Other definitions for erato that I've seen before include "Poetic Muse ; eighteen (anag.)" , "Lyric poetry's muse" , "Clio's sister" , "goddess" , "Poet's inspiration" .)