A function I must host in desert (5)
I believe the answer is:
sinai
'desert' is the definition.
(Sinai Desert is an example)
'a function i must host' is the wordplay.
'function' becomes 'sin' (mathematical function).
'must host' means one lot of letters goes inside another (inserted letters are hosted or welcomed in).
'sin'+'i'='sini'
'a' put into 'sini' is 'SINAI'.
'in' acts as a link.
(Other definitions for sinai that I've seen before include "Ten Commandments mountain" , "Mt. where Moses received 10 Commandments" , "11 Across peninsula" , "Deserted area" , "Mountainous Egyptian peninsula" .)