Bloodsucking insect’s low around sierra desert (8)
I believe the answer is:
mosquito
'bloodsucking insect's' is the definition.
'low around sierra desert' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'around' means one lot of letters goes inside another.
'sierra' becomes 's' (phonetic alphabet: alpha, bravo, charlie etc.).
'desert' becomes 'quit' (I've seen this before).
's'+'quit'='squit'
'moo' enclosing 'squit' is 'MOSQUITO'.
(Other definitions for mosquito that I've seen before include "Flying biting insect" , "Insect, the female of which sucks blood" , "possible disease carrier" , "Insect that transmits malaria" , "Kind of blood-sucking tropical gnat" .)