One's usually needled to learn Jennifer's missing first half (7)
I believe the answer is:
conifer
'one's usually needled' is the definition.
(conifers typically have needles)
'learn jennifer's missing first half' is the wordplay.
'learn' becomes 'con' (synonyms).
'missing first half' suggests halving the letters.
'jennifer' cut in half is 'ifer'.
'con'+'ifer'='CONIFER'
'to' is the link.
(Other definitions for conifer that I've seen before include "Fir or larch, for example" , "Evergreen tree" , "The tree keeps its leaves" , "Eg, pine, fir or cypress" , "Type of tree" .)