Satisfied concerning poet's rhythm (5)
I believe the answer is:
metre
'poet's rhythm' is the definition.
(poetic metre)
'satisfied concerning' is the wordplay.
'satisfied' becomes 'met' (as in 'met the conditions of his contract').
'concerning' becomes 're' (both can mean on a particular topic).
'met'+'re'='METRE'
(Other definitions for metre that I've seen before include "Approx. thirty nine point three seven inches" , "Rhythmic arrangement of syllables in verse" , "1.094 yards" , "100cm" , "Poetic rhythm" .)