Film’s star to support a cause, though not at first (4)
I believe the answer is:
lead
'film's star' is the definition.
'lead' can be an answer for 'star' (synonyms). I'm not certain of the 'film's' bit.
'support a cause though not at first' is the wordplay.
'support' becomes 'lea' (I can't explain this - if you can you should give a lot more credence to this answer).
'a cause' becomes 'MD' (MD is an example).
'though not at first' means to remove the first letter (the first letter is removed).
'md' with its first letter taken off is 'd'.
'lea'+'d'='LEAD'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for lead that I've seen before include "spearhead" , "Toxic, malleable, metallic element" , "One with starring role" , "Leash -- Pb" , "Live" .)