Occasion Her Maj will avoid one giving out lines (6)
I believe the answer is:
prompt
'occasion' is the definition.
(occasion can mean to cause or prompt)
'her maj will avoid one giving out lines' is the wordplay.
'her maj' becomes 'ER' (Elizabeth Regina ie Her Majesty Queen Elizabeth).
'will avoid' indicates named letters should be taken away.
'one giving out lines' becomes 'prompter' (someone giving an actor line prompts).
'prompter' with 'er' taken out is 'PROMPT'.
(Other definitions for prompt that I've seen before include "Quick, without delay" , "Encourage" , "Incite - punctual" , "Bang on" , "effective in expedition" .)