Quote bareheaded, ancient, Celtic priest, looking towards the stars - it'll get you going (8)
I believe the answer is:
diuretic
'it'll get you going' is the definition.
(a diuretic makes you urinate or 'go')
'quote bareheaded ancient celtic priest looking towards the stars' is the wordplay.
'quote' becomes 'cite' (synonyms).
'bareheaded' means to remove the first letter (the 'head' of the word is removed).
'ancient celtic priest' becomes 'druid' (priest in ancient Celtic religion).
'looking towards the stars' is a reversal indicator (in a down clue, letters go up).
'druid' with its initial letter removed is 'ruid'.
'cite'+'ruid'='citeruid'
'citeruid' back-to-front is 'DIURETIC'.
(Another definition for diuretic that I've seen is " Increasing urination".)