Unseen, let tan develop (6)
I believe the answer is:
latent
'unseen' is the definition.
(thesaurus)
'let tan develop' is the wordplay.
'develop' indicates anagramming the letters.
'let'+'tan'='lettan'
'lettan' is an anagram of 'LATENT'.
(Other definitions for latent that I've seen before include "Existing but not yet active or visible" , "Potential not realised" , "Not presently active" , "Potential, dormant" , "Hidden (talent)" .)