Screen one Zulu bitten by fat reptile (7,6)
I believe the answer is:
monitor lizard
'reptile' is the definition.
(monitor lizard is a kind of reptile)
'screen one zulu bitten by fat' is the wordplay.
'screen' becomes 'monitor' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'zulu' becomes 'z' (phonetic alphabet: alpha, bravo, charlie etc.).
'bitten by' means one lot of letters goes inside another (I've seen this in other clues).
'fat' becomes 'lard' (I've seen this before).
'i'+'z'='iz'
'iz' going into 'lard' is 'lizard'.
'monitor'+'lizard'='MONITOR LIZARD'
(Other definitions for monitor lizard that I've seen before include "Beast" , "Reptile" .)