Underwater craft let loose, not immediately obvious (6)
I believe the answer is:
subtle
'not immediately obvious' is the definition.
(I've seen this before)
'underwater craft let loose' is the wordplay.
'underwater craft' becomes 'sub' (submarine).
'loose' is an anagram indicator (letters are loosened from their original positions).
'let' is an anagram of 'tle'.
'sub'+'tle'='SUBTLE'
(Other definitions for subtle that I've seen before include "Slight and difficult to analyse, as distinction may be" , "Delicate, difficult to describe" , "Capable of making fine distinctions" , "Very fine, difficult to perceive or appreciate" , "Slight and difficult to detect, like some humour" .)