Enquire about a small ruler (3)
I believe the answer is:
ask
'enquire' is the definition.
(to enquire is to ask or question)
'a small ruler' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ruler' becomes 'K' (abbreviation for king in chess).
'a'+'s'+'k'='ASK'
'about' is the link.
(Other definitions for ask that I've seen before include "Invite (to a party)" , "Put a question" , "Seek an answer" , "Issue invitations" , "Inquire" .)