First of twins desperate for the other to be troublesome (6)
I believe the answer is:
thorny
'be troublesome' is the definition.
(I've seen this before)
'first of twins desperate for the other' is the wordplay.
'first of' suggests taking the first letters.
'desperate for the other' becomes 'horny' ('the other' is old-fashioned slang for 'sex').
The initial letter of 'twins' is 't'.
't'+'horny'='THORNY'
'to' is the link.
(Other definitions for thorny that I've seen before include "Troublesome or difficult - having spikes?" , "Troublesome or difficult, like briars?" , "Problematic; spiny" , "Trying" , "Problematic; having spines" .)