Specific word in song briefly captivating trekker? (7)

'specific word' is the definition.
(specific term within a wider category)

'song briefly captivating trekker?' is the wordplay.
'song' becomes 'hymn' (religious song).
'briefly' means to remove the last letter.
'captivating' is an insertion indicator (I've seen this in other clues).
'trekker?' becomes 'pony' (animal which treks).
'hymn' with its last letter taken off is 'hym'.
'hym' enclosing 'pony' is 'HYPONYM'.

'in' acts as a link.