River - Tiber - new height at new appearance (7)
I believe the answer is:
rebirth
'new appearance' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'river tiber new height' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'new' is an anagram indicator.
'height' becomes 'h' (abbreviation in geometry).
'tiber' anagrammed gives 'ebirt'.
'r'+'ebirt'+'h'='REBIRTH'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for rebirth that I've seen before include "spiritual enlightenment" , "Renascence" , "Revival; reincarnation" , "Start of a new life?" , "Spiritual renewal" .)