Flirtation, losing head, leads to marriage (8)
I believe the answer is:
alliance
'marriage' is the definition.
(I've seen this before)
'flirtation losing head' is the wordplay.
'flirtation' becomes 'dalliance' (both can mean a casual relationship).
'losing head' means to remove the first letter.
'dalliance' with its first letter removed is 'ALLIANCE'.
'leads to' acts as a link.
(Other definitions for alliance that I've seen before include "match?" , "Kind of partnership with canaille" , "League - treaty" , "Union, association" , "Group acting together for common cause" .)