Altar? I'm fearful of marriage (7)
I believe the answer is:
marital
'of marriage' is the definition.
(I've seen this before)
'altar? i'm fearful' is the wordplay.
'fearful' is an anagram indicator.
'altar'+'im'='altarim'
'altarim' anagrammed gives 'MARITAL'.
(Other definitions for marital that I've seen before include "related to union" , "Maybe husband's" , "to do with life together" , "Of the nature of marriage" , "alliance" .)