Complex sentence: In other Upanishads, the concept of Bhakti is indirectly voiced in the form of "Upasana", but here, leaving no room for guesswork, it is explicitly mentioned.
Split into simpler sentences: In other Upanishads, the concept of devotion (or Bhakti) is indirectly voiced in the form of "Upasana". But here, without leaving any room for guesswork, it is explicitly mentioned.