... gem tense old man smuggled into Australia (5)
I believe the answer is:
topaz
'gem' is the definition.
(I know that topaz is a type of transparent gem)
'tense old man smuggled into australia' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'old man' becomes 'pa' (both can mean father).
'smuggled into' means one lot of letters goes inside another.
'australia' becomes 'oz' (informal name for Australia).
'pa' going into 'oz' is 'opaz'.
't'+'opaz'='TOPAZ'
(Other definitions for topaz that I've seen before include "Shade of dark yellow" , "Shade of yellow -- gem" , "White/colourless mineral used as a gemstone" , "Semi-precious stone of various colours, especially yellow" , "Precious stone - dark yellow colour" .)