Composer reaching height? Rather unlikely (7)
I believe the answer is:
tallish
'rather unlikely' is the definition.
(tall can mean unlikely as in a tall tale)
'composer reaching height?' is the wordplay.
'composer' becomes 'tallis' (I've seen this before**).
'reaching' says to put letters next to each other.
'height?' becomes 'h'.
'tallis'+'h'='TALLISH'
(Other definitions for tallish that I've seen before include "Rather lofty" , "Consider" , "rather large" .)