Hymn: type of music sea fog conjured up (4,2,4)
I believe the answer is:
rock of ages
'hymn' is the definition.
(I've seen this before)
'type of music sea fog conjured up' is the wordplay.
'type of music' becomes 'rock' (**).
'conjured up' is an anagram indicator (I've seen 'conjuring up' mean this).
'sea'+'fog'='seafog'
'seafog' with letters rearranged gives 'ofages'.
'rock'+'ofages'='ROCK OF AGES'
(Other definitions for rock of ages that I've seen before include "song" , "Hymn" .)