Film director's short with Romeo who should know his lines (6)
I believe the answer is:
angler
'film director's short with romeo who should know his' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'lines' is the wordplay.
I cannot really see how this works, but
'lines' could be 'l' (abbreviation as in lines of poetry) and 'l' is present in the answer.
The remaining letters 'anger' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for angler that I've seen before include "One hoping for a catch" , "Fisher with hook and line." , "Hook and line fisherman" , "One proverbially exaggerating" , "A rod and line fisherman" .)