One diver comes up holding cold chunk of glacier (7)
I believe the answer is:
iceberg
'chunk of glacier' is the definition.
'one diver comes up holding cold' is the wordplay.
'one' becomes 'i' (Roman numeral).
'diver' becomes 'grebe' (diving bird).
'comes up' is a reversal indicator.
'holding' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'grebe' backwards is 'eberg'.
'i'+'eberg'='ieberg'
'ieberg' placed around 'c' is 'ICEBERG'.
(Other definitions for iceberg that I've seen before include "threat to ship" , "danger at sea" , "Four-fifths are under water" , "Growler" , "One with a cold manner" .)