In play, one posh actor needs to speak like Cockney (4,4,7)
I believe the answer is:
drop ones aitches
'speak like cockney' is the definition.
(a typical Cockney accent drops H's)
'in play one posh actor needs' is the wordplay.
'in play' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'i'+'posh'+'actor'+'needs'='iposhactorneeds'
'iposhactorneeds' with letters rearranged gives 'DROP ONES AITCHES'.
'to' is the link.
(Other definitions for drop ones aitches that I've seen before include "Pronounce carelessly" , "should Cockney enunciate thus?" , "Show lack of class" , "lack aspiration, so to speak" .)