Some crocodiles around open-air pool (4)
'open-air pool' is the definition.
(I've seen this before)
'some crocodiles around' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'around' shows that the letters should be reversed in order.
'odil' is hidden in the letters of 'crocodiles'.
'odil' in reverse letter order is 'LIDO'.
(Other definitions for lido that I've seen before include "Venice's beach island facing the Adriatic", "Swimming and leisure area", "Swim here", "bathing-beach", "---- di Jesolo, Italian Adriatic resort".)