Desperate need to consume crack, heading for altar (7)
I believe the answer is:
engaged
'heading for altar' is the definition.
(due to get married at the altar)
'desperate need to consume crack' is the wordplay.
'desperate' is an anagram indicator (I've seen this in other clues).
'to consume' means one lot of letters goes inside another.
'crack' becomes 'gag' (both can mean a joke).
'need' with letters rearranged gives 'ened'.
'ened' placed around 'gag' is 'ENGAGED'.
(Other definitions for engaged that I've seen before include "Bound by promise" , "Not free" , "Formally committed to marriage" , "Bethrothed" , "Taken on - betrothed" .)