In the UK, non-commissioned officer ranks start to crack (6)
I believe the answer is:
uncork
'crack' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Maybe you can see an association between them that I don't see?
'in the uk non-commissioned officer ranks start' is the wordplay.
'in the' indicates putting letters inside.
'non commissioned officer' becomes 'NCO' (abbreviation).
'start' says to take the initial letters.
The initial letter of 'ranks' is 'r'.
'nco'+'r'='ncor'
'uk' enclosing 'ncor' is 'UNCORK'.
'to' is the link.
Can you help me to learn more?
(Other definitions for uncork that I've seen before include "Clear a bottleneck" , "Open (Chablis, say)" , "Open (a wine bottle)" , "Release the stopper" , "open tent?" .)