Seven steep rocks on peak in Tanzania (6)
I believe the answer is:
septet
'seven' is the definition.
(synonyms)
'steep rocks on peak in tanzania' is the wordplay.
'rocks' is an anagram indicator.
'on' indicates putting letters inside (as in clothing 'on' a person).
'peak in' suggests taking the first letters (in a down clue, the letter at the top of the word).
The initial letter of 'tanzania' is 't'.
'steep' with letters rearranged gives 'sepet'.
'sepet' enclosing 't' is 'SEPTET'.
(Other definitions for septet that I've seen before include "Snow White's companions?" , "A set of seven" , "Group of seven players" , "Band of seven players" , "Work for seven players" .)