Musical theatre, at least initially (7)
I believe the answer is:
lyrical
'musical' is the definition.
(thesaurus)
'theatre at least initially' is the wordplay.
'theatre' becomes 'Lyric' (Lyric Theatre in London).
'initially' indicates taking the first letters.
The initial letters of 'at least' is 'al'.
'lyric'+'al'='LYRICAL'
(Other definitions for lyrical that I've seen before include "To wax - is to enthuse" , "In poetic words, song-like" , "Highly enthusiastic" , "Expressing deep emotion" , "carried away" .)