Turning 100, Catholic husband becomes sex symbol! (5)
I believe the answer is:
notch
'sex symbol' is the definition.
(as in a notch on a bed post)
'turning 100 catholic husband' is the wordplay.
'turning' shows that the letters should be reversed in order.
'100' becomes 'ton' (ton can informally mean a hundred).
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'husband' becomes 'h'.
'ton' back-to-front is 'not'.
'not'+'c'+'h'='NOTCH'
'becomes' is the link.
(Other definitions for notch that I've seen before include "Cleft" , "small score?" , "Nick made in something as a record" , "Carved incision" , "V-shaped nick" .)